CDC Flip-Flops on Paradigm Shift to SARS-COV-2 Transmission via Aerosols

By Lambert Strether of Corrente.

Patient readers, I’m a little pressed temporally, and so this post may be a little less coherent than usual. Let me jump in–

As readers know, I stan for aerosol transmission as the primary tranmission mechanism for SARS-COV-2; that is, singing, shouting, talking, even breathing, all of which give rise to small virus-bearing particles that float indefinite distances (aerosols), as opposed to coughing or sneezing, which give rise to larger particles that fall, pulled down by gravity (droplets), after travelling one or two meters (and also accumulate on surfaces, which are then to be wiped). As I wrote back in May:

From the beginning of the #COVID19 pandemic, we’ve been washing our hands, masking up, cleaning surfaces, and social distancing. These measures have worked (especially masking), but now we know more. There’s mounting evidence that airborne transmission indoors is a key — perhaps the main — pathway to SARS-COV-2 transmission. In this post I want to look at why that’s so, give examples, and suggest a simple heuristic to stay safe. Material like this might also be used to inform public policy (here; here) by reducing superspreader events in enclosed spaces like churches (airborne transmission via singing), restaurants (loud talking, especially if room is noisy), bars (ditto), nursing homes (shouting[1]), gyms (grunting), meat-packing plants (shouting), call centers (talking), offices generally (air conditioning), and other hot spots, but working that policy out is not the object of this post (see here for engineering controls for airborne transmission, and here for covid-proofing public spaces).

I took this view because of case studies, given in the post, for which aerosol transmission could give an account, and droplet tranmission could not. (Subsequently, actual transmission of viable viral material through the air was demonstrated in two hospital studies, posted in Links.)

So it was with great interest that I received the following mail from alert reader Chi Gal in Carolina, saying that the CDC had finally updated its guidance to support aerosol transmission. (I quote Chi Gal to give her a hat tip for taking point on this topic in comments). Chi Gal wrote:

Subject: Finally, the CDC updated its guidance! (link from ChiGal).

On September 18, just 7 months in. Maybe now we have a shot at getting this thing under control.

First, I checked the Twitter, and found this headline from the Los Angeles Times: “CDC says coronavirus spreads mainly in the air, through respiratory aerosols and droplets.” OMG! Happily and excitedly, I went to the CDC website, where this notice appeared:

I took a screenshot and sent it to Chi Gal:

“Did the version you saw have this at the top:”


And that, dear readers, is all that anybody knows (at least out here in the Great Unwashed). Here is the CDC guidance that appeared and disappeared. From the Hill, the CDC guidance that appeared and disappeared:

“There is growing evidence that droplets and airborne particles can remain suspended in the air and be breathed in by others, and travel distances beyond 6 feet (for example, during choir practice, in restaurants, or in fitness classes),” the agency had written. “In general, indoor environments without good ventilation increase this risk.”

“These particles can be inhaled into the nose, mouth, airways, and lungs and cause infection,” the deleted guidance said. “This is thought to be the main way the virus spreads.”

OMG, indeed.

* * *

So what happened? I can come up with two possible accounts: The institutional and the political. (Of course, “the institutional is the political,” as Carol Hanisch did not say, but let us keep the two separate, if only as objects of study).

Institutionally, the conflict between proponents of droplet transmission, and the proponents of aerosol transmission, is part of the process known in the history of science as a paradigm shift, as discussed here in “Don’t ‘Trust the Science,’ Trust Science While You Hone Your Critical Thinking Skills.” (Here is an example at NC of a paradigm shift in river restoration). This excellent thread from aerosol scientist Jose-Luis Jimenez shows such a shift in action. It’s worth reading in full, but I’ll pull out the key tweets. Jimenez begins by critiquing the now-vanished CDC guidance:

(His basic point, explained over several tweets, is that CDC cannot, at the same time, claim that Covid spreads by “inhalation” of droplets, because droplets are too big. (I mean, if I hack up a loogie, you’re not inhaling it, right? And so on down to the smallest possible size of droplet.) But here is the key point:

And why does Chapin rule out airborne infection?

“A great relief to most persons to be freed from the specter of infected air” means “people might panic” (!). And here we are:

You can read the rest of the thread for today’s institutional players: Basically, WHO and the CDC people have been droplet proponents for a long time, and, since science proceeds by conflict — which is why “Trust the science!” doesn’t work when applied uncritically — they need to be persuaded, or, if worst comes to worst, defenestrated in the usual way: One funeral at a time, as Max Planck said. To be fair to the medical profession, they have proceeded with far greater dispatch than physicists!

So, one explanation for the new CDC guidance being pulled is that, institutionally, the old guard won.

Politically, you know the already congealing narrative. Here is the classiest, least hysterical example of it.

But there are a few problems with Mitchell’s Tweet.

First, Democrat operative and medical ethicist[1] slash private equity/venture capitalist partner Zeke Emanuel is eminently quotable but foolish: As we have explained at length, “the science” is not always a matter of “facts” but of the paradigms we use to give an account of facts. WHO, for example, does not regard aerosols as the primary transmission path for Covid as a fact at all. Zeke’s embarrassing Neera on this, and he should do better.

Second, I have checked CBS, The Hill, NPR, Politico, WaPo, a second article in WaPo, and the Wall Street Journal. None of them suggest the guidance was “squelched” for “political considerations.” The New York Times says explicitly that it wasn’t:

Experts with knowledge of the incident said on Monday that the latest reversal appeared to be a genuine mistake in the agency’s scientific review process, rather than the result of political meddling. Officials said the agency would soon publish revised guidance.

Of course, one never knows when the blow may fall; anonymous sources could contradict the Times tomorrow. Nevertheless, Occam’s Razor would suggest than when we have an institutional account, we don’t need to invent a political one.

Third, and ironically, if there was, anybody doing the squelching — in today’s impoverished analytical environment — would be able to say “I did trust the science! I checked with WHO!!”

So that is the state of play on the CDC’s aerosol guidance as of today. Let’s see what they come up with!


[1] It would be unfair to call Emanuel a homicidal maniac based on a single article entitled “Why I hope to die at 75,” even if from today’s Water Cooler we note that Japanese master painter and woodblock printer Hokusai “understood the structure of animals, birds, insects and fishes, and the life of grasses and plants” only at age 73, dying 16 years later at the age of 89.

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About Lambert Strether

Readers, I have had a correspondent characterize my views as realistic cynical. Let me briefly explain them. I believe in universal programs that provide concrete material benefits, especially to the working class. Medicare for All is the prime example, but tuition-free college and a Post Office Bank also fall under this heading. So do a Jobs Guarantee and a Debt Jubilee. Clearly, neither liberal Democrats nor conservative Republicans can deliver on such programs, because the two are different flavors of neoliberalism (“Because markets”). I don’t much care about the “ism” that delivers the benefits, although whichever one does have to put common humanity first, as opposed to markets. Could be a second FDR saving capitalism, democratic socialism leashing and collaring it, or communism razing it. I don’t much care, as long as the benefits are delivered. To me, the key issue — and this is why Medicare for All is always first with me — is the tens of thousands of excess “deaths from despair,” as described by the Case-Deaton study, and other recent studies. That enormous body count makes Medicare for All, at the very least, a moral and strategic imperative. And that level of suffering and organic damage makes the concerns of identity politics — even the worthy fight to help the refugees Bush, Obama, and Clinton’s wars created — bright shiny objects by comparison. Hence my frustration with the news flow — currently in my view the swirling intersection of two, separate Shock Doctrine campaigns, one by the Administration, and the other by out-of-power liberals and their allies in the State and in the press — a news flow that constantly forces me to focus on matters that I regard as of secondary importance to the excess deaths. What kind of political economy is it that halts or even reverses the increases in life expectancy that civilized societies have achieved? I am also very hopeful that the continuing destruction of both party establishments will open the space for voices supporting programs similar to those I have listed; let’s call such voices “the left.” Volatility creates opportunity, especially if the Democrat establishment, which puts markets first and opposes all such programs, isn’t allowed to get back into the saddle. Eyes on the prize! I love the tactical level, and secretly love even the horse race, since I’ve been blogging about it daily for fourteen years, but everything I write has this perspective at the back of it.


  1. Louis Fyne

    Don’t reinvent the wheel!

    The entire world should just copy whatever the Koreans and Taiwanese do—to the extent politically-culturally-financially possible.

    /rant over—but not holding by breath as the US still can’t even make enough N95 masks domestically. ugh.

    1. Artifex

      Taiwan does not recommend N95 masks for civilians. It does however manufacture enough high quality medical-type masks for its own population and rations them. That is a very simple measure that has resulted in good results. If there are any asymptotic spreading going on it is fully suppressed just widespread mask usage and limited social distancing.

    2. juliania

      I continue to ignore mainstream press and go with what is being told us by the folk at the University of California San Francisco medical center who have posted a series of seminars weekly that discuss these matters of aerosols and droplets and what actual hospital studies are showing in a calm manner. Each week they demonstrate what clinical trials are showing and what we know and don’t know based on those studies. And always it has been geared to the importance of mask wearing when out of the home, so I do that. One of those studies proposed that virus intensity was possibly cumulative, so the mask, even a home made one, which I have worn from the getgo, may allow a few virus particles to penetrate, and this could be a positive effect, since with the little we actually know about resistance to this new virus, that may give at least partial immunity. I can’t praise them highly enough, as they managed to keep that encouraging demeanor, even while San Francisco has been in the midst of those terrible California fires.

      With all that in mind, it’s still a good idea to social distance when encountering friends on the street. In a store, (which is my only other activity these days) usually I have a cart in front of me, so masked and with that, I feel very safe. I think my home is a safe environment so I don’t wash hands excessively, just when I come home or have got them dirty in the garden, that sort of thing. And my hope is the virus will eventually run out of singers and shouters and calm itself down to a sniffle or a cough somewhere down the line, having met up with a gentle lady covid virus and gotten itself happily and safely married off.

      If all of this doesn’t work, c’est la vie.

      PS. If anyone thinks I am taking it all too lightly, I am not. It scares the daylights out of me sometimes, as I have dear ones in harm’s way. But they are soldiering on, and so must I.

      1. rd

        Since the spring, I have assumed that the risk is in layers and the solutions need to be in layers.

        Very bad risk: coughing, sneezing etc. droplets within 6′ – high viral load, you can get very sick

        Solution: mask, sanitize surfaces, stay the hell away and get out, wash hands, get good ventilation/filtering, change clothes, etc. View as major bio-hazard.

        Medium risk: talking, singing etc in poorly ventilated space, low to medium viral load depending on exposure duration, may not get really sick, could just be asymptomatic

        Solution: mask, leave or improve ventilation/filtering, stay at least 6 feet away (preferably 10 or more), wash hands, sanitize high-touch surfaces

        Low risk: in groups of people outside or in well ventilated/filtered space, may not get really sick, could just be asymptomatic

        Solution: mask, keep several feet apart, wash hands periodically.

        I have also assumed that an asymptomatic person can still generate a reasonably high viral load in droplets or aerosols, so I don’t assume anybody is safe, even in a low risk environment, unless I know a lot about the precautions they are taking or they have just been tested and found negative.

        1. Fiery Hunt

          I’m just gonna suggest you move the “medium risk”” list to the ultra high risk.
          Pretty sure it’s been discussed here several times (thanks, Lambert!)…
          As far as I know, none of the super-spreader events were caused by coughing or sneezing…

    3. Temporarily Sane

      It’s clear that the super duper “advanced” countries are incapable of doing that under present neoliberal circumstances. If they could they would.

      The system is held together by duct tape, chewing gum and string with a soundtrack of batsh*t insane gibberish (Conniving Russkies! Devious Chinamen! Pedo satanists! Orange Putlers!) dialed up to 11 and everyone running around in circles yelling and screaming at each other.

      (Maybe replacing a functioning society with Twitter wasn’t such a great idea…)

  2. steve

    I think they fear the consequences of acknowledging airborne transmission and their objections are just dressed up in “science”. What exactly do they fear though? People are much more likely to avoid enclosed spaces if they fear the “specter of infected air” so a deterioration of the economy? A slowing of the spread and decrease of infected? Sticking to droplet transmission is all about the consequences of admitting to aerosol transmission. But what is their calculus?

  3. Samuel Conner

    This may be an uncharitable, or perhaps tin-foily, thought but, if I have understood the emphasized text in that tweet, perhaps there is debate within the CDC about how best to motivate public compliance with what are currently regarded to be the most effective protective measures.

    Perhaps “accurate description of the state of the science” is not always the most effective (in the sense of “most likely to slow the progress of an epidemic”) public health communication strategy.

    1. albrt

      Sure, just lie to the public based on short term goals and everything will be fine! Fauci did it and look how well that turned out.

  4. allan

    None of them suggest the guidance was “squelched” for “political considerations.”

    Well, this implies that there’s a good chance that it was:

    CDC’s messages to public were slowed by extensive reviews by other agencies, sources say [Stars and Stripes]

    A few months ago the Trump administration put in place new procedures to review Centers for Disease Control and Prevention guidelines related to COVID-19, subjecting hundreds of proposed documents and publications to questioning by officials across the administration, according to people familiar with the matter.

    Since then, officials from across the government have been weighing in on almost every document or website related to COVID-19 advice that the agency has published, the people said. The full extent of the role other federal agencies are playing in shaping CDC recommendations hasn’t been previously reported.

    While the additional scrutiny rarely resulted in major changes to the CDC’s publications, the people said, it delayed guidance to nursing homes, schools, houses of worship and businesses, sometimes for weeks, as infections soared across the U.S. …

    Authorities involved in the reviews included the White House Coronavirus Task Force, OMB and OIRA, the Department of Homeland Security, the Labor Department, the State Department and the Department of Education, among others, as well as CDC’s parent organization, the Health and Human Services Department. …

    Pence. OMB. Chad Wolf. Eugene Scalia. Mike Pompeo. Betsy DeVos.
    A Dream Team of public health experts you would want to have a finger in every CDC guidance pie.
    What could possibly go wrong? Oh, wait …

    … One person said the outside perspective was sometimes helpful, but it slowed down publication. For example, guidance for nursing homes went through at least 10 rounds of review, delaying it for several weeks. …

    A few weeks here, a few weeks there, pretty soon you’re talking about real carnage.

    1. Lambert Strether Post author

      The transition from “squelch” to “delay” does involve some goalpost moving, doesn’t it? (“the additional scrutiny rarely resulted in major changes”). That said, I don’t see a reason to regard the CDC’s guidance as worthwhile, a priori, given their track record on testing. Personally, I wish the Federal government spoke with one voice, but given the givens, I’m glad that many agencies were involved instead of either no others or the Executive alone. Finally, the assumption of your comments seems to be to “trust the[se] experts” in a matter than has enormous political consequences for the entire population. For something like measles or food poisoning, yes. For a pandemic, which necessarily involves the entire body politic? No. That is something I am simply not willing to do.

      1. allan

        From Despair at CDC after Trump influence: ‘I have never seen morale this low’:

        The Trump administration’s bungled response to the coronavirus pandemic and its subsequent efforts to meddle with recommendations from the Centers for Disease Control and Prevention (CDC) are taking a substantial toll on the nation’s foremost public health institution.

        In interviews with half a dozen current and former CDC officials, they described a workforce that has seen its expertise questioned, its findings overturned for political purposes and its effectiveness in combating the pandemic undermined by partisan actors in Washington. …

        Others at the CDC said political interference has been taking place from the first days of the pandemic. On one incident management call, an official who listened in said Redfield talked about how he had been instructed by Vice President Pence to change CDC guidelines on the size of public gatherings to come in line with the White House coronavirus task force recommendations. …

        I don’t see how one can criticize the CDC guidance on anything when we don’t know what the guidance
        was originally supposed to be.

  5. Wally

    For what it is worth, the person whose advice I would follow above all others is Osterholm. He has said from the beginning that aerosol needs to be considered as a factor and has pointed out that the word ‘mask’ is a generic, non-specific description and so saying that ‘masks’ prevent spread is not a sound statement without a description of said masks. (He has caught hell for that, by the way, from people who should know better).
    I believe he has been right all along on both of these issues, but reason is not much respected these days.
    It is sad to see the CDC be so backward for so long. I saw a study way back at the beginning of this which traced the spread of Covid infection in a restaurant to the exact seating patterns of patrons based on the airflow of the mechanical system. That’s as clear a proof as you could ask for aerosol spread.
    The simple rule to follow is: Don’t breathe another person’s backwash.

    1. Lambert Strether Post author

      > I saw a study way back at the beginning of this which traced the spread of Covid infection in a restaurant to the exact seating patterns of patrons based on the airflow of the mechanical system.

      That study, with diagram, is at NC here, along with the China bus study that persuaded me, also with diagram.

  6. The Historian

    Let’s see what I remember from my years of working with asbestos, beryllium, radioactive materials and other hazardous materials.

    1) You can inhale particles of various sizes, but the particle has to be less than 10 microns to get into your lungs. This is why people who work around hazardous materials wear masks or respirators. Even those people who paint your car wear masks or respirators.

    2. Particles of less than 10 microns can travel great distances in air. Consider the smoke from wildfires which has particulate sizes of between 2 and 5 microns. CDC has a good powerpoint that explains the issues: It is also the reason you put on a mask or respirator before going into a closed building containing friable asbestos or in a closed area when you are working with beryllium.

    3. Particles from coughs come in a great range of sizes from less than a micron to over 15 microns. Sneezing will produce a greater range of sizes because it is more explosive.

    Am I reading the tweets by Jiminez wrong? Because as an aerosol scientist, he should know that coughs and sneezes do produce “droplets” in the 1-15 micron range. And people are washing their hands often and washing things that come into their homes, in addition to wearing masks and social distancing. I do not get where he things that people can’t do both and aren’t doing both at the same time.

    Here’s a good video of a cough:

    So just from what I know from working with fine particulates for years, I’m still going to maintain my social distance and wear my mask – no matter what the CDC comes up with! I wonder who will will this battle – the politicians or the actual scientists of CDC!

    1. rd

      My understanding is that the lungs are actually pretty poor filters, so the pure virus (typically < 1 micron) is more likely to be exhaled than trapped by the lungs.

      The big droplets (say bigger than 10 microns or so) will probably get trapped in the nose/throat and start the infection there from which it can progress down into the lungs over time.

      The aerosols in the 1 to 10 micron or so range can get inhaled into the lungs and then get trapped along with whatever virus is hanging onto them. That infection can start in the lungs, although it may not have a high initial viral load.

      So a decent cloth mask can both prevent a lot of virus from being expelled from someone, reducing the viral load in droplets within 6 foot or smaller aerosols that can drift on air currents. The same mask on a receptor can absorb a bunch of the aerosol or droplets, reducing the viral load that can get into the nose, throat, and lungs. Glasses, safety glasses, and face shields can reduce potential entry route through the eyes.

      A person causes warm air immediately around them and so they create a rising column of air. So emitted aerosols can initially rise until the air cools. That is very different from the parabolic arc to the floor of a droplet and is why aerosols can end up 10 feet or more from the emitting person and stay in the air for several minutes.

  7. parisblues

    The spectacle of daily whiplash at the CDC feels like the reports of that EgyptAir crash in which two pilots appeared to have been battling it out in the cockpit for control of the stick. Looks like the “Trump-u-Akbar” faction has won this round.

  8. Temporarily Sane

    I get that SARS2/Covid-19 is a new condition and that science and medicine can’t perform miracles and figure everything out in a few weeks or months. But the flip-flopping and mixed/contradictory messaging going on here around very basic issues (is the virus airborne? are masks effective?) is unreal.

    The CDC, WHO and governments in many “advanced” nations are doing a great job showcasing their utter incompetence and inability to learn from those countries that actually have this thing under control. Just providing the public with reliable and consistent information is too much for them to handle.

    We are lucky that Covid-19 is relatively tame as far as transmittable diseases go. If it was more virile, like bubonic plague or Ebola, there really would be rotting corpses piling up in the streets.

  9. VietnamVet

    It was a political/ideological decision within the neoliberal West to make money and wait the pandemic out until a for-profit Pharmaceutical Industry vaccine is available next year – ignore the deaths unless hospitals are overwhelmed and then impose a lockdown.

    Donald Trump says the vaccine will be available before the election in order to stay in the White House. For 99% of Americans there are no alternatives other than isolating oneself from others until sometime in the far future or wear masks and gloves if out in public to lower risks, ignore the fear, and play Russian Roulette – get infected or not. A functional public health system that works like in Asia and South Pacific nations is not going to happen. Really rich, valuable, Westerners set up real time coronavirus testing and isolation bubbles like the White House and the NBA.

    The powerful, by refusing to pay for public health, decided to do nothing but cannot admit it. As a result, messaging from Washington DC is contradictory and highly confusing. This has ended up with two opposing partisan camps that say a) the pandemic is no big deal or b) the 200,000 deaths are Donald Trump’s fault (even though he is a symptom of the corrupt deadly system not the cause).

    1. Jackson

      Trump is not concerned with the population at large. His warped mind is in “herd ” It won’t work, Sweden is hiding a lot of data. Trump wants to exceed 6 million deaths so he can join Hitler, Mussolini, Pol Pot and Stalin as his heroes and peers.

    2. Lambert Strether Post author

      > The powerful, by refusing to pay for public health, decided to do nothing but cannot admit it.

      Blue State governors like Cuomo and Newsome — presumably where Democrats would look for executive talent — were not exactly powerless. By statute, most public heath power is at state and local levels.

      As some NC commenter said recently: Trump is the stench. He’s not the rot.

  10. John Zelnicker

    Lambert – You repeated Jimenez”s tweets at “And why does Chapin rule out airborne infection” and then at “”A great relief to most persons to be freed from the specter of infected air” means “people might panic” (!). And here we are:” just below that.

    Did you mean to include a different tweet?

  11. drumlin woodchuckles

    This won’t be related to the subject. But threads die in a day or less so I will put this here in the chance that it may be seen.

    This is something for the tenured academic professors and other recreational leftists who pretend that looting is “not” violence and who chatter on about how looting is “revolutionary”.
    After looking at this video and then thinking about the recreational looting which BLM and Antifa indulged in, I think a Trump victory looks more likely every day.

    1. Zamfir

      The big bad is recirculation. Many large-scale HVAC systems suck air from rooms, cool it, then send it back to all rooms (recirculate). They mix in “enough” outside air to keep it fresh, but not too much.

      That way, you don’t have to cool (or heat) an entire flow of outside air. The recirculated air is usually closer to the target temperature than the outside air.

      For the corona,you want maximum amounts of outside air. And you don’t want to spread the old air throughout the building. So no recirculation, all of it outside air.

      Usually, there is a simple setting to accomplish this, but someone has to be aware and change that setting. And then the coolers (or heaters) have to be large enough to handle the full flow. If not, you might get away with filtering the recirculated air, but then the fans have to be strong enough to handle the extra resistance of a filter – often they are not.

      My impression is that this is real barrier, even the required modifications are fairly minor. People are willing to switch the settings without much pressure, even if gives them a much high energy bill later on. But once it gets to modifications to a system, you get a different business dynamic, where you have to really justify the spending upfront (even if the amount is similar to that power bill). And that justification is vague, unless there are hard requirements from authorities.

      As a visitor, you cant really tell what’s happening there. A building may be recirculating lots of air (so bad), or ventilating lots of fresh air (very good), but it looks and feels mostly the same to a visitor.

      Apart from the recirculation, there is the simple amount of ventilation airflow that the system can handle. That is often on the minimal side, because over-designed systems are first in the block for budget cuts during construction. And upgrading the flow rate is often not a minor investment, but a major overhaul. The alternative is to open windows, but if the building is designed for closed windows, there is often no easy way to change that.

      Another issue are “split” airconditioners, more often found in smaller buildings. Here the air conditioner unit pumps around air within a room, and another system circulates cooling fluid (but not air). This is probably safer than a recirculating central system, but less good then a central system that brings in fresh air. There were some documented cases were these split units seemed to promote Corona spread within a room, by creating extra air flow. But this might be coincidental – split units go together with bad ventilation rates (little outside air), so the problem may be more in the bad ventilation than in the split units .

      1. ChiGal in Carolina

        Good info, thanks. And thanks to Lambert for locating the now gone (if imperfect since droplets cannot be inhaled) new guidance.

        I was just totally undone by seeing the CDC finally acknowledge airborne transmission only to have it disappear before my very eyes:
        OMG indeed.

        1. Zamfir

          I realize that I should add a clarifier: there is a common-sense logic that recirculation is bad, and people should surely act on that logic.

          But I don’t think it is definitely proven, for the corona virus. AFAIK, it it is definitely possible that the virus rarely survives a trip through an HVAC system, or that it typically dilutes to such levels that the risk is neglibly low.

          I feel that there is a fundamental difference here between engineers and doctors. Doctors are trained in “first do no harm”. They assume (with good reason) that interventions will have unknown negative side effects. And they also know that many plausible good effects don’t work out in reality. So they say, do not prescribe an intervention until you know that it helps.

          They see “increased ventilation” (or shutting down recirculation) as an intervention – they want hard evidence that it helps, before they prescribe it.

          Engineers are trained in risk reduction – if it is plausible that something can go wrong wrong, you assume that it will go wrong for sure, until you have evidence of the opposite. We tend to see “Corona spread through the HVAC system” as a risk – you should mitigate it unless you can show that the risk is low.

          Both are reasonable thinking shortcuts, but here they give opposite answers on what counts as “careful”. Both sides tend to see the other as reckless.

    2. Lambert Strether Post author

      I believe impactful — the case study of the Chinese restaurant was HVAC-related — but I am too lazy and rushed to dig out links.

      I do not recall a definitive study. I do believe HVAC manufacturers are thinking through measures for proper filtration and perhaps UV. IIRC, Clive urges that there be a constant intake of air from the outside, for dilution, as opposed to recirculating the same air. I believe that humidity plays a part as well (I do not remember whether high or low). I believe that the same factors apply to personal AC as well as building HVAC, but in my own personal practice I do not use AC. I have a fan going at all times, and like a good Victorian, leave the window open (or at least a door).

      Purely on intuition, I believe that circulation patterns within buildings matter as well; that the virus will concentrate in certain areas much as plastic accumulates in the Pacific “garbage patch.”

      HVAC readers please chime in!

      1. Clive

        Yes, that’s it in a nutshell: Outside air not recirculation. ASHRAE recommends running on 100% outside air (a good precis of the guidance is here and it’s worth reading the whole thing for context and additional information)

        And correct, too, regarding humidity, from the ASHRAE guidance:

        CONTAIN: Control indoor humidity
        • Install humidity sensors, update control sequences, and add equipment or components to maintain indoor RH between 40 and 60 percent
        • Disable discharge-air temperature reset for multiple-zone VAV systems during humid weather
        • Ensure hot-water heating system is enabled to provide reheat for humidity control, if necessary

        (my emphasis)

        As for running, or not running A/C, it all depends. If your ventilation system relies on running the HVAC for inducing outside air (sealed windows or otherwise low airflow from outside), you should run the A/C in order to bring in air from outside the space and to exhaust air from inside it. And relying on open windows may well be insufficient: if there’s little air movement (no breeze or it is blowing in the wrong direction to make an open window bring in appreciable air) then a fan, be it stand-alone or as part of the HVAC unit itself, may be essential.

        Most importantly of all, if you’re in a humid climate then if the outside air is 70-90% RH then that is what you’re bringing into the unconditioned space. In the heating season, you might reduce that if you’re subsequently warming the air — but that can create very low humilities (before I sealed up my house better with improved insulation, a damp but cold day where the heating ran a lot created indoor RH of 20% sometimes, for example). And obviously during the cooling season, without A/C, you’re sitting in all that high humidity, it won’t simply disappear all by itself without mechanical cooling.

      2. Zamfir

        I had another comment, but it disappeared. I am not an HVAc specialist, but I do work in a somewhat adjacent field.

        Ventilation (bringing fresh air) is IMO key. It’s robust: it will work under a range of situations.

        Filtering should work as well, but it is less robust. People have to install the right filters, put them in right, no leak paths, replace them. There might be some subtle effect that filters fail on. The extra pressure drop may reduce ventilation rates. And you cannot check this from the outside. Best is visible ventilation – open windows etc

        UV is further down my wish list. Perhaps it works if done right, but few people know what is right, and there is an industry of snake oil salesmen selling magic lamps. I suspect that most actually installed UV installation would be window-dressing.

        Humidity is even more suspect, IMO. UV at least has hard evidence of killing the virus, under correct circumstance. The humidity evidence is mostly vague correlations that might be due to a myriad of confounding factors. Perhaps it has an effect, but I wouldn’t put any bet on it. Just make the humidity pleasant.

        A fan in your room does little, corona wise. If you share the space with others, the fan may help spread the particles better between you, or it might dillute the air better and decrease risk. But I guess that is minor. Body heat already moves air enough.

        I am personally not very worried about “accumulation zones”, for that same reason. You can have still zones in buildings, but the mere presence of person (who moves and heats) tends to end such still zones quickly.

        Also, most evidence suggests that you need a fair bit of exposure for the corona to hit – that’s why the distancing does help. It would be difficult to have a local build-up comparable to near a person.

        For comparison, think about body odours. Those spread similar to aerosols. The smell of a person dimishes quickly if you away from them. It doesn’t build up in unoccupied corners. Unless you have smelly clothing lying around, a fresh source itself. But you can get body smell out of a ventilation duct, of there are people on the other side of that system

      3. rd

        The number of air volume changes per hour is critical. You can have the best filtration and fresh air input percentage in the world, but if you only do a half room volume change per hour, it is largely irrelevant. Many older buildings are designed to only do 1 to 3 air changes per hour. 5 or 6 is generally recommended for good quality indoor air, and some experts recommend minimum of 9 in aerosol-transmitted pandemics.

        My target when I drop HEPA air purifiers in a room is size them for minimum 5-6 air changes per hour while also keeping occupancy at a relatively low density so that people are spaced apart while still wearing masks unless they are otherwise protected from other people. I prefer multiple smaller air purifiers so that the travel distance for potentially contaminated air is much smaller which can reduce the potential for it to be inhaled by someone before it is filtered.

        When I walk into a building, I look hard at the geometry and occupancy of the building. A big building with lots of ceiling room (big box stores, gymnasiums etc.) with low density of people means that a lot of dilution is available – you just need to be careful about getting up close and eprsonal with people. Many bars are small and cramped with poor ventilation with people talking, so they are a major risk even if you are more than 6 feet from someone.

        If you are in a building with good air volume, 5+ air changes per hour with MERV-13 or better filtration, and good fresh air addition, then you are probably in pretty good shape. Good luck finding that information out.

  12. PlutoniumKun

    Thanks for this Lambert, as always NC delivers.

    Its been very clear to me since the very beginning of this pandemic that there were fundamental scientific flaws in the approach of the scientific community in the west. I found it a bizarre sight to see public health authorities successfully combat the spread in China, SK, HK, Taiwan and Vietnam while everywhere else floundered, and yet we were firmly assured that hand washing was more important than masks and that travel restrictions don’t work, when it was very clear that this was not the approach of successful Asian authorities. It always seemed to me that we were seeing one thing on our screens (everyone in Asia fully masked up going about their business), while hearing another thing from our public authorities. The early data from South Korean churches made it abundantly clear that aerosol transmission had to be considered as a major source of spread, if not the primary source.

    My own theory was that western authorities were far too hung up on the flu heuristic and seemed incapable of pulling themselves away from that paradigm – while those countries in Asia with recent experience of SARs were treating Covid as it should be treated – as a new virus with more in common with the common cold than flu, at least in terms of transmission. I’ve lost count of the times I’ve seen scientists (especially, it should be said, those in the ‘hard’ mathematical sciences it should be said), stating that ‘this is not the flu’, while rigidly applying models derived from the flu in their assessments. It says a lot that ‘youtube authorities’ like Peak Prosperity and Dr. John Campbell were consistently calling things right while the medical establishment was floundering. I suspect much has to come with their more ‘hands on’ perspective – pathologist and nursing specialist.

    While scientific errors are understandable and inevitable – this is how science proceeds, I find the manner in which many authorities (and numerous independent scientists who should know better ) have dug themselves into their intellectual holes to be quite unforgivable. This doesn’t just apply to transmission, it has badly affected the approach to treatment, in particular the refusal of the main public bodies to give advice on matters like vitamins. This has been a catastrophic failure of the scientific method and it has its roots in the way public science in particular works. I’m tired if hearing people say things like ‘believe the science’, when (as I did a few days ago) I hear a professor of physics from Oxford on the radio telling us all that masks don’t work. He has a model apparently that tells us this. (In my experience, btw, physicists are the branch of science who are most likely to get things badly wrong* when they stray out of their own area of expertise).

    I’ve lost faith in the ability of our authorities to own up to mistakes, but I do think that in the longer term, Covid will be written up in books as a classic case of paradigm failure (back in the 1980’s, as a callow undergrad, this was a favourite topic of many of my lecturers, but for some reason its fallen out of favour.) I briefly studied geomorphology – a subject which has had more than one fundamental paradigm shift – and it was fascinating to read about the process by which discredited theories hung on for decades more than they should have, because of the deep reluctance of professors of whatever to admit that half their lives work had been wrong. Anthropology and archaeology are similar subjects which were plagued with incorrect heuristics that led to libraries full of books which, shall we say, haven’t aged well. Even the hardest sciences are guilty of this (I’d strongly recommend the book The Trouble with Physics by Lee Smolin for anyone who thinks that the hard sciences are less prone to this than the softer ones). There are already mountains of papers and books out there on the deep problems within medical sciences and incorrect use of data and poor epistemology, no doubt many more will be added on the subject of Covid. And thats before we even start to scratch at the surface on the malign influence of Big Pharm and politics.

    *before anyone pulls me up on this, I don’t consider economics to be a science.

    1. Ignacio

      Haha PK, I think I have contrarian arguments on your take that I have made before. As you, I found frustrating that, for instance, the WHO had been sticking too fiercely with the droplet + fomites route. IMO, this is absolutely not a scientifically-driven mistake but a 100% politically driven one. Admitting at the very beginning airborne transmission would make the WHO fear such a reaction by the public at large on travelling, gathering etc. that might have stopped to a halt, 0, nada, activities such as air-travelling, public transport, and many other activities that involve any gathering. By the way it would have been very useful halting international flights at the very beginning not when it was already too late.

      Science at the frontier of knowledge is anything but a consensus. Instead is a constant challenge. In our life we had never faced a challenge like that SARS CoV 2 has resulted: a completely new virus which is transmitted with ease human to human (the Chinese failure to communicate that early enough in the epidemic was one of the biggest mistakes if not the biggest of all mistakes). This was totally new so we were all caught off guard. Asian countries benefited from previous experience, not as challenging as SARS2.0, but the lethality of SARS1.0 was an eye opener. This proves that experience helps.

      1. PlutoniumKun

        I don’t think we are as far apart on this as it seems – there are still obviously major ‘unknowns’ about how the virus transmits (I still don’t understand, for example, why in the UK London has been most lightly affected while the north of England is bad), but it was very obvious to me that a focus on fomites and hand washing was important – but is not the most important thing, and this should have been apparent very early, as it was to the South Koreans and HKers.

        I first suspected something was wrong when WHO and national authorities were saying that international travel was safe, quoting a number of previous studies – but even the quickest glance at those studies showed that they were of very limited relevance (mostly they were about land movements in Africa). Anyone who has read William McNeill’s classic study on historic pandemics knows that international trade routes are what spread infectious diseases, whether the old Silk Routes, or trans Siberian railways in the last 19th Century. Its just common sense (an underestimated factor in science).

        So yes, its politics in the broadest sense. But I do think that epistemology is very important – I think public bodies, even the most rigorous – have been very slow and very bad at looking objectively at all the information from a wide variety of sources, and the perspectives of different specialities, and focusing on providing useable information for the public and decision makers. Its one thing to make a bad call at the beginning of an epidemic – this is inevitable. What I find so disappointing is that so many scientists and public officials have been so slow to reverse course and admit they were wrong. Far too many bad recommendations have been allowed to stay ‘official’ long after science has moved on.

        I should state, btw, that I’m no expert at all in the topic of viruses or disease, my interest is mostly in scientific knowledge and the application of knowledge to practical policy (a research topic of mine a long time ago). So I would bow to any specialists knowledge on the behaviour of the disease. What I despair about is the failure to dispassionately apply what we know (and what we know we don’t know) to what we should be doing in a manner consistent with what we know about risk (in particular, fat-tailed risk).

        1. Ignacio

          Yes, I don’t see we have such a distant position PK that is why i wrote a laugh in the very first line of comment very much confident I can relax arguing with you about this. I very much like reading your comments just in case you doubt it.

      2. Lambert Strether Post author

        > As you, I found frustrating that, for instance, the WHO had been sticking too fiercely with the droplet + fomites route. IMO, this is absolutely not a scientifically-driven mistake but a 100% politically driven one.

        A good reason not to put a single agency like the CDC in charge of everything. Sadly!

      3. drumlin woodchuckles

        I earnestly request my fellow readers to consider the possibility that the WHO considered the virus outbreak a ” god-given” opportunity to spread this disease across the world in order to disadvantage non-China against China in China’s ” New Global Hegemon” race.

        And that when the WHO thinks it can get away with it, it will continue perverting science and knowledge to slow the rest of the world’s response to corona in order to keep advantaging China.

  13. Ignacio

    I am very much a ‘fan’ of airborne spread as from the very beginning spreading events strongly suggested it. Yet, I am lately very much concerned about direct transmission through hugging/kissing or fomites-led by bottle/glass sharing etc which could, IMO, explain why Madrid is again a Covid-19 epicentre by the behaviour of the many who do not understand what is social distancing (Chapin has a point). I am talking about spreading events in places such as beaches and parks between younger who meet in what in Spain is called ‘botellones’ which are more or less large late-night gatherings in parks and other public open spaces (forbidden in many places but difficult to control) with drinkings (shared fomites there) smoking (more fomites there) and music (no fomites I think) and where airborne transmission is quite unlikely. That is why in Madrid, new restrictions include the closure of parks.

    This is not to say that airborne is quite possibly the most important route but by no means it should be ruled out the role of other transmission routes which in some circumstances could be more important. The most affected districts in Madrid are, of course densely populated working-class type with incidences above 1000 new cases per 100.000 habitants for the last 14 days. Social distancing there is difficult and each house is a densely populated enclosure where airborne transmission is very much facilitated. This is not to say that more wealthy districts are covid-free with incidences at about 500/100.000 that probably benefit from less in-house and work site transmission but these are not free from botellones and other social gatherings that involve close contact + fomites sharing.

    1. PlutoniumKun

      I think its pretty clear that cultural issues are very important in its spread, and southern European countries seem more vulnerable in this way. Someone joked recently that the reason London is much less affected than the north of England is because ‘nobody talks to each other there’, and there may be some truth to this.

      And I think yes, there has been a problem with public education in that the focus has been on ‘the rules’ and not on ‘here is how a virus spreads’. I’ve seen people genuinely try their best to ‘follow the rules’, while simultaneously do what I consider to be very dangerous things, and I think this is a result of confused messaging. I’ve certainly seen people share from bottles and glasses quite casually while also maintaining social distance.

      1. Ignacio

        ‘a problem with public education’

        That is IMO a major problem. Disinformation and conspiracies haven’t helped.

      2. Zamfir

        My impression is that the ‘paradigm’ stuff is made worse by the need for public education, and for authorative rules/guidelines.

        Those rules and education are by necessity simplified (or even oversimplified). Without the enormous machinery of subtlety and exceptions that scientists can usually employ to paper over their differences.

        I have seen this with the masks. You get some medical scientist who has good reasons to be skeptical about masks -are they good enough, will people use them right, where is the hard evidence? If this was an internal debate among faculty, they might gradually shift their position, by some steady flow of arguments and evidence.

        But now they are pushed to make a hard simple statement early on, for use in the outside world. And then they are on record as “masks don’t help”, which makes it harder to change position (even to oneself)

        Also, they encounter the oversimplified position of opponents -magic mask people who promise that everything goes away with a mask mandate. Or blatantly interested people who want autoritive rules in their favour- touring car operators who want to hear that masks will allow them to fill their buses again. HVAC suppliers who want mandates on high ventilation rates. This makes it easier to dismiss other views.

        Sometimes scientists find agreement by mumbling “it’s complicated, new information, a bit of this but also that, laymen don’t understand the subtleties”. That helps to save face . It’s how you do paradigm stuff without waiting for the funerals. But it is horrible public communication.

        1. PlutoniumKun

          But now they are pushed to make a hard simple statement early on, for use in the outside world. And then they are on record as “masks don’t help”, which makes it harder to change position (even to oneself)

          A key problem I have with this issue is the failure to apply good principles of decision making.

          There are two well established principles that are relevant to any policy decision in a pandemic:

          1. The irreversibility principle. When faced with two options, both of which seem equally justifiable, the one you choose should be the one most easy to reverse (or the one that will do least damage if you have to reverse it).

          2. The precautionary principle. As Nicholas Nassim Taleb has been shouting for a while, when faced with fat tailed risks (i.e. risks with low probability, but high impact), you should always apply the precautionary principle. An outbreak of a novel virus is as clear an example as you’ll find of a fat tailed risk.

          When you apply these two principles to the question of widespread adoption of mask, and to international travel, it is absolutely clear that the wrong decisions were made in the west. If it turns out everyone was wrong about masks, what was the harm? Slightly more problems in getting hospital masks (and that could have been addressed by strict rules on sales). Plus the cost of a few dollars to individuals in buying one.

          Travel is even worse. The question should not have been ‘can we justify shutting down international travel?’ The question should have been ‘can the international travel industry prove they can operate safely without spreading the virus?’.

          I would argue that there were plenty of other wrong decisions based on the above two principles, including the failure to advise vulnerable populations to boost their immune systems (especially with Vitamin D), and the failure to look into using existing vaccines (such as the TB vaccine) to help boosting the immune properties of larger swathes of the population.

          My personal belief is that a significant number of scientists could not step back from the heuristic of requiring the levels of ‘proof’ that you would apply to a new drug or vaccine to see that a more cost/benefit risk based approach is far more applicable to a wide range of anti-pandemic actions. Taleb of course could see this from the very beginning, the question is why more scientists didn’t adopt the same approach.

          1. Code Name D

            My personal belief is that a significant number of scientists could not step back from the heuristic of requiring the levels of ‘proof’ that you would apply to a new drug or vaccine to see that a more cost/benefit risk based approach is far more applicable to a wide range of anti-pandemic actions.

            This is a bit like blaming cooks because they couldn’t fix a car. It’s not in their skill set and its not their job.

            Science is about giving us the best understanding possible of the observable universe. What you are talking about are matters of policy – not science. Masks had been a staple in dealing with pandemics sense the 1800. The problem was that politics, anti-science, and political narcissism all conspired to not only make poor decisions – but a refusal to learn from ones mistakes and adapt to new information & conditions.

            1. PlutoniumKun

              I wish that was true, but its not. Many of the policies were driven by scientists and many politicians were operating on the basis of faulty advice by scientists. It was scientists who said that masks should not be used and it was scientists who said that air travel should not be shut down. It was scientists who allowed highly faulty papers to be published in prestigious publications.

              Simply saying ‘its not our fault, the politicians made their decisions’, is not good enough.

    2. Lambert Strether Post author

      > very much concerned about direct transmission through hugging/kissing or fomites-led by bottle/glass sharing etc

      Excellent point. In none of the aerosol studies I can remember were behaviors similar to hugging/kissing practiced, at least not on the scale of, say, beer pong on university campuses.*

      In my view, one of the major failings in the West, top to bottom, has been the positive avoidance of our evolution-granted narrative skills, such that we simply do not watch what people do and think through the implications, something often practiced in the humanities, at least until the post-modernists came along. This was certainly the case among the wise fools in our university administrations, who through years of on-campus life seem somehow to have not seen that young people party, and do, er, other things afterwards. Yes, I get the money angle, but I think they were just blind to the behaviors of those not in their immediate purview, those not of their class (“Yes, of course they will comply, we sent out a memo”). Similarly, the wonderful thing about the studies with seating charts and maps** is that the logic of human-to-human transmission becomes instantly clear to the imagination, and can be applied elsewhere.

      * I do not know whether the Korean Christian churches practice “the kiss of peace” or not.

      ** One would think that fomites advocates would be able to produce similar literature but if so I have not seen it. Readers?

      1. Clive

        The top-down hierarchical approach to public health is a huge impediment not only to adoption of whatever public health policies are decided on but also nicely squishes out the essential feedback loop which needs to run something like:

        Expertise -> Policy -> Communication -> Adoption -> User Experience -> Expertise

        And then goes into reverse. By which I mean, once the adopters of the public health policy actually start to understand it and follow it it is they who are then the experts. They know what works for them and what doesn’t, what went well and what went badly, what helped and was either ineffective or even counterproductive. There is a need for the “original” experts (the ones who helped develop public health policy) to then defer to the emergent group of (now inherently superior) experts — the people who have been following the public health policy (you and I, in other words).

        This, of course, seldom happens in public health. The original experts not only fail to grasp how their expertise becomes flawed through a failure to understand where it might be limited or ineffective in the field but actually are prone to both blaming of public policy adherents for any failures and also disparaging the people for failing to remember just where they should be sitting in the expertise pecking order.

        1. Ignacio

          Talking about learned expertise and absence of bottom-up feedback. I can talk about my own experience about contagions in a night beach gathering including my son who resulted one being infected (amongst some more ) weeks ago. He knew about social distancing but he also knew that summer is time for fun. I am almost certain that direct contact and fomite-led through sharing bottles were the main routes in that event (even If I avoided embarrassing questions but tried indirect detective-style questioning).

          Regarding the feedback this is probably on of the biggest failures of the HC management since the feed-back mechanism for this is very well-known: contact tracing. In my experience there are mechanisms that break that feed-back mechanism. One is fear to stigma (many refuse to communicate as they don’t want to be confined, damn it! and I have witnessed that very directly) this prevents further testing and stops contact tracing when needed. Second is insufficient contact tracing effort and this is a big problem in Madrid, where the contact tracing teams –too few–have been easily overwhelmed. One of the measures taken in Spain has been to reduce confinement from 14 to 10 days as this would reduce fear to stigma significantly while the risk of spread might increase only marginally by the reduction. I agree with that, and this means that there is some feed-back functioning. Another issue would be lack of responsibility in social terms (social stupidity). Many still look at this as a question of personal risk and decisions, as if further disease spread wouldn’t affect them personally.

  14. Hayek's Heelbiter

    Microdroplets less than 100th of millimetre in size may spread the coronavirus.
    Research in Japan shows microdroplets can remain in the air for 20 minutes in enclosed spaces.

    The end-stage pathway to infection of the droplet and contact transmission routes has always been assumed to be via self-inoculation into mucous membranes (of the eyes, nose and mouth). Surprisingly, no direct confirmatory evidence of this phenomenon has been reported, e.g. where there have been: (i) follow-up of fomite or droplet-contaminated fingers of a host, self-inoculated to the mucous membranes to cause infection, through the related disease incubation period, to the development of disease,
    The W.H.O. has resisted mounting evidence that viral particles floating indoors are infectious, some scientists say.
    Ventilation systems in schools, nursing homes, residences and businesses may need to minimize recirculating air and add powerful new filters. Ultraviolet lights may be needed to kill viral particles floating in tiny droplets indoors.
    Don Milton, a medical doctor and an environmental-health professor at the University of Maryland, compares larger droplets “to the spray from a Windex dispenser” and the smaller, air-borne particles (aerosols) “to the mist from an ultrasonic humidifier.” Clearly, it’s enough to merely step back—distance—to avoid the former, but distancing alone would not be enough to avoid breathing in the latter.

    The disagreement got heated enough that earlier this month, hundreds of scientists around the world signed a letter, pleading with the WHO to acknowledge these smaller particles as an extra mode of transmission and to update its guidelines accordingly. Some experts I spoke with told me that they had been trying to convince the WHO to take the possibility of airborne transmission since March, and that the open letter was borne out of frustration about lack of progress.
    Results: Of the 110 cases examined, 27 (24.6%) were primary cases who generated secondary cases. The odds that a primary case transmitted COVID-19 in a closed environment was 18.7 times greater compared to an open-air environment (95% confidence interval [CI]: 6.0, 57.9).
    (Proceedings of the National Academy of Sciences of the United States of America)
    Face covering prevents both airborne transmission by blocking atomization and inhalation of virus-bearing aerosols and con-tact transmission by blocking viral shedding of droplets. On the other hand, social distancing, quarantine, and isolation, in conjunction with hand sanitizing, minimize contact (direct and indirect) transmission but do not protect against airborne transmission. With social distancing, quarantine, and isola-tion in place worldwide and in the United States since the be-ginning of April, airborne transmission represents the only viable route for spreading the disease, when mandated face covering is not implemented.
    Poor ventilation in confined indoor spaces is associated with increased transmission of respiratory infections [1]. … COVID-19 is thought to be primarily transmitted via large respiratory droplets, however, an increasing number of out-break reports implicate the role of aerosols in COVID-19 outbreaks.

    In residential houses and apartments, normal practices (e.g. segregating infected individuals, OPENING WINDOWS AND DOORS, and using portable air-cleaning devices when practical) to ENSURE HEALTHY INDOOR AIR, should stay in place at any moment.

  15. Zamfir

    My impression is that the ‘paradigm’ stuff is made worse by the need for public education, and for authorative rules/guidelines.

    Those rules and education are by necessity simplified (or even oversimplified). Without the enormous machinery of subtlety and exceptions that scientists can usually employ to paper over their differences.

    I have seen this with the masks. You get some medical scientist who has good reasons to be skeptical about masks -are they good enough, will people use them right, where is the hard evidence? If this was an internal debate among colleagues, they might gradually shift their position, by some steady flow of arguments and evidence.

    But now they are pushed to make a hard simple statement early on, for use in the outside world. And then they are on record as “masks don’t help”, which makes it harder to change position (even to oneself)

    Also, they encounter the oversimplified position of opponents -magic mask people who promise that everything goes away with a mask mandate. Or blatantly interested people – touring car operators who want to hear that masks will allow them to fill their buses again. HVAC suppliers who want mandates on high ventilation rated. This makes it easier to dismiss opponents.

    Sometimes scientists find agreement by mumbling “it’s complicated, new information, a bit of this but also that, laymen don’t understand the subtleties”. That helps to save face . It’s how you do paradigm stuff without waiting for the funerals. But it is horrible public communication.

  16. Jeremy Grimm

    I think public health — especially a public health problem like a new virus — requires a heuristic combined with a practical approach — practical in the sense that one should apply what seems to work, or has worked elsewhere, and doesn’t cost a great deal or cause collateral harm [greater harm than a lack of supply because of our stupid just-in-time inventory system of goods produced far far away]. Let science poke around for the verifiable ‘correct’ answers on its own time — which raises the question what have our scientists been doing all this time that they know so little about how SARS spreads? The Corona virus is a new virus but it isn’t exactly the first of its kind, nor is it likely to be the last.

    How could the U.S. be caught so bare-assed after spending billions on Homeland Security, studying potential biological warfare threats, and watching viruses spreading in other countries across the oceans but not so very far away by air? The problem of having little production of simple items like face masks is not newly discovered. The remarkably slow and half-assed response to a lack of inventory and lack of domestic production of something like medical face masks showcases a combination of plain incompetence and willful profiteering by our Big Money Corporate powers.

  17. Susan the other

    We probably have cute avuncular old Bob Woodward to thank for this latest confection by the CDC. It’s just political reaction. Trump knew C19 was spread in droplets and aerosols, per the “innocent” interview with Woodward. The democrats chose a politically quick time to smear Trump for covering up the aerosol connection and downplaying masks. Woodward is as big a sleaze ball as Trump is a clodhopper. The bottom line is that Trump was hapless – he was told in no uncertain terms not to cause panic. And he then proceeded to be such an unconvincing speaker (because he couldn’t BS his way through it) that everybody… panicked. The good news is we all panicked logically and put our masks on and stayed 6 feet apart. There is nothing in the universe as important as common sense. I wonder why nobody studies it.

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