1/ Understanding CDC’s updated guidance that aerosols are the main mode of SARS-CoV-2 transmission CDC recently updated its guidance. It is worded in a confusing way & this makes it a little self-contradictory. This is causing some confusion, so I’ll try to explain it here
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7/ EPA gives us the summary of a lot of research on deposition of inhaled particles in the respiratory tract at https://www.epa.gov/pmcourse/particle-pollution-exposure … So basically particles up to ~100 um can be inhaled. Larger particles cannot.pic.twitter.com/i25qw30oKW
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8/ Droplets are defined as particles that fall in 1-2 m & infect by impact on eyes, nostrils, or mouth Let’s review the history of "droplet" for disease transmission (we are very close to submitting a paper on this history, so it is fresh in my mind):https://twitter.com/linseymarr/status/1278036517890347008 …
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9/ “Droplets” in disease transmission arose from the work of Carl Fuggle in 1890s. Using collection on agar plates, he showed that bacteria expelled from the mouth were observed close but not far from the subjects.https://zenodo.org/record/1686460#.X2gmwmhTljE …
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10/ Charles Chapin, President of the American Public Health Association, wrote a seminar book in 1910. Correctly concluded that close proximity led to respiratory infection, and that social distance worked to reduce it. That’s where social distance comes from, and it works
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11/ But why does social distance work? Unfortunately, he incorrectly attributed this observation to “sprayborne” transmission, due to Flugge’s droplets. And the said that “airbone” transmission was nearly impossible. A fateful error. Chapin's book: https://archive.org/details/sourcesmodesofin00ch …
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12/ Chapin: “former ideas [that] diseases may be air-borne are entirely erroneous; that most diseases are [...] spray-borne only for two or three feet, a phenomenon which after all resembles contact infection more than it does aerial infection as ordinarily understood.”pic.twitter.com/6ndVxeguSZ
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13/ Let’s take that in for a moment. According to Chapin, SPRAY-borne infection, due to Flugge’s droplets, explains “contact infection," i.e. infection in close proximity (2-3 ft). AIR-borne does not.
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14/ Chapin says not enough evidence to rule out airborne infection. But still states that airborne infection is nearly impossible (“we are warranted to discard it as a working hypothesis”).
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15/ Why does he discard airborne infection? Because he new that social distance + hygiene worked and: “It is impossible, as I know from experience, to teach people to avoid contact infection while they are firmly convinced that the air is the chief vehicle of infection.’’pic.twitter.com/PrWUPPf4Bx
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16/ Chapin’s position becomes the new paradigm, and it has reigned successfully until now. Airborne transmission is resisted, as it has for COVID-19. Only very contagious diseases (+TB) are proven, as evidence becomes undeniable.
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17/ Leading to *confusion of this artifact of history (only v contagious accepted as aerosol), with a law of nature.* Stated in arguments like "if COVID-19 was aerosol, spread would be much larger, R0 would be much larger, we would see long-range transmission" etc. All false
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18/ Wells in 1934 at Harvard studies the droplet vs aerosol problem. He correctly determines that the aerosol-droplet separation is at ~100 um. https://academic.oup.com/aje/article-abstract/20/3/611/280025?redirectedFrom=fulltext …pic.twitter.com/vF5yGiJiCr
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19/ Wells clearly defines droplet and airborne infection, consistent with Flugge and Chapin (who had defined “droplet infection proper”). (Note that “proper droplets” are larger than 100 um, and thus we know now they are not inhalable).pic.twitter.com/7BYAbruZRO
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20/ Back to the present: CDC defines droplet infection as caused by projectiles E.g. see slide from the presentation by Dr. Butler (CDC's Deputy Director for Infectious Diseases) at recent workshop of the US National Academies of Sciences, Engineering, & Medicine (pres. #5):pic.twitter.com/yIZlFQVBGE
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21/ As recently as this
@NASEM workshop on 26-Aug-2020, CDC was stating (via Dr. Butler) that SARS-CoV-2 was spread mainly through “droplets that can land in the mouth, noses, or eyes of people” https://www.nationalacademies.org/event/08-26-2020/airborne-transmission-of-sars-cov-2-a-virtual-workshop …pic.twitter.com/hBULb5VIjI
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22/ Consistent with historical practice at CDC. E.g. as summarized in 2007 paper from Y. Li’s group (https://onlinelibrary.wiley.com/doi/10.1111/j.1600-0668.2007.00469.x …), leader in disease transmission. “Droplets [..] are propelled a short distance through the air, and deposited on the host’s [eyes, nostrils], or mouth”pic.twitter.com/WkAE9qvRph
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23/ The same is true for the WHO, e.g. their famous video showing the ballistic “angry birds” flight of the droplets (https://twitter.com/WHO/status/1244258441880797184 …):pic.twitter.com/ftKA8RFPws
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24/ And this is clearly what the latest WHO brief says (9-Jul): “droplets can reach the mouth, nose, or eyes” while “within 1 metre”. This is consistent with all the historical definition of droplets. https://www.who.int/publications/i/item/modes-of-transmission-of-virus-causing-covid-19-implications-for-ipc-precaution-recommendations …pic.twitter.com/uveW7BVRWY
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25/ It is also discussed in the WHO brief that aerosols could be inhaled and lead to infection. Note that the word “inhale” is only present in this document for aerosols, and never for droplets.
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26/ Also note the dismissive tone about aerosols in the 9-Jul
@WHO brief: “hypotheses… theories…” which continues later in the document. In my experience, nobody has read that document and concluded that aerosols were major. Possible maybe, but very uncertain.Show this thread -
27/ So it is extremely clear what droplets & aerosols are for disease transmission, per history, CDC, WHO Droplets are ballistic projectiles, fall to the ground 1-2 m, infect by impact on eyes / nostrils / mouth Aerosols float in air for longer times, and infect by inhalation
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28/ CRITICAL reminder : - to be droplet & fall to ground, need > 100 um. - Being a projectile also requires > 100 um (when talking, a projectile requires > 300 um). See this thread and refs therein:https://twitter.com/jljcolorado/status/1292880342227984385 …
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29/ And to be inhalable, need < 100 um. So there is a clear boundary between droplets and aerosols at 100 um For much of the 20th Century, droplets and aerosols were separated at 100 um for infection purposes in guidance and papers, with references tracing back to Wells (1934)
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30/ But at some point in the later 20th Century, an error is introduced, for reasons we will explain when our history pre-print is available (https://twitter.com/linseymarr/status/1278036517890347008 …) As a consequence of the error, health orgs start stating that the droplet-aerosol boundary is at 5 um
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31/ The 5 um error happened long enough ago, that for many people active today, 5 um is the only definition they may have ever known. And it is still an error.
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32/ Fauci admitted recently that the 5 um aerosol/droplet boundary is an error:https://twitter.com/adamhamdy/status/1304120806943928329 …
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33/ The
@NASEM workshop also concluded that 100 um is the correct aerosol-droplet boundary, and that 5 um is incorrectpic.twitter.com/g0CV0UxlmK
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34/ Indeed 5 um makes no sense. Particles of 5 um can float in the air for 30 min. This is from the “Aerosol’s 101” tutorial in the CDC webpage (http://www.cdc.gov/niosh/topics/aerosols/pdfs/Aerosol_101.pdf …):pic.twitter.com/IE5NTcQTm4
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35/ However, 5 um error is widely adopted and repeated in many papers and guidance from CDC and WHO for decades. For example in Klompas et al (2020) in JAMA: https://jamanetwork.com/journals/jama/fullarticle/2768396 … & in very recent paper from key members of the
@WHO IPC committee:https://aricjournal.biomedcentral.com/articles/10.1186/s13756-020-00779-6 …Show this thread -
36/ Sometimes, Wells (1934) is wrongly cited for the 5 um aerosol/droplet boundary, even though he clearly states > 100 um We have found no other paper that would support why to define 5 um as anything special in the droplet-aerosol range. (Pls send to me, if you have any)
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