Understating Airborne Covid-19

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My title is slightly misleading but meant to tell you what this essay is about. I want to talk about recent reports that SARS-CoV-2, the virus that causes Covid-19, is airborne.

The concept of airborne in thinking about pathogens is probably the single most misunderstood thing in epidemiology, not by epidemiologists but by regular people. It is also probably the most evocative, and stupifying. Recently, the word “airborne” has been used in discussions of Covid-19, and this led to many extreme reactions. Like this:

Input: Bla bla bla Covid-19 bla bla bla airborne.

Output: All the credible experts have agreed that Covid is airbone! it is no longer spread by contact, but now it is airborne! The Fauci mutation probably made it airborne! And so on!

SARS-CoV-2 is spread by shedding from an infected person’s respiratory system and getting into a new host’s respiratory system via droplets of mucus that go from hand to hand, hand to mouth, mouth to hand, mouth or nose to surface, surface to hand then hand to mouth, etc. Human upper respiratory bodily fluids (snot, etc.) get on stuff and then people touch stuff and then it get into their respiratory system. This is how most cold and flu infections are passed on, generally. That is not airborne spread.

Among all the many viruses that give us colds or the flu — the many strains of influenza, rhinovirus, coronaviruses other than SARS-CoV-2, etc. — this is how infection happens.

Again, this is not airborne spread. It might be airborne in your head, because you imagine someone sneezing, into the air, droplets of virus-containing spittle and snot flying around in the air, and since that stuff flies through the air it must therefore airborne. But that is not what airborne means, and the distinction is important.

There is probably a certain amount of true airborne transmission in any of the above mentioned categories of virus, including the flu and more common colds. But it is rare enough that these diseases are not said to be airborne.

So what is airborne then, if it is not simply flying snot particles?

Airborne spread requires several things to be true often enough that an observable number of cases were spread in this way. First, the virus must be aerosolized. This means that the virus is embedded in a very small gobs of snot, perhaps near 5 microns, droplets that are small enough to be suspended in the air. Larger drops will fall out of the air, these smaller drops will float in the air like they were part of the air. They act like a gas in the air.

The droplets also have to be small enough to get into the parts of the respiratory system that the virus targets, which for SARS-CoV-2 is not too much of a limiting factor since it likes to inhabit the upper respiratory tract. But, since it also can invade the lungs, there would be the possibility that airborne transmission would be more associated with a more serious infection, if and when it happens.

Airborne spread also requires that the virus can live in the air long enough to get to its target. The longer the virus can live in the air, the worse of a problem it is because it can travel farther, through ventilation systems, down hallways, etc. There is no evidence that SARS-CoV-2 does that like, say measles, the king of airborne infection does it. Whatever environmental conditions are experienced by this suspended droplet have to NOT kill the virus. IV light kills SARS-CoV-2, so it is not going to get far during the day, outside, when the sun is out. SARS-CoV-2 might like certain humidity levels. None of this is really known for SARS-CoV-2, but it is a virus of type that we generally know about. Other forms of coronavirus are known to survive a while in the air, so that may pertain here. Be cautious in reacting to what you hear though. Detecting a virus some distance away from a sneeze does not mean that the virus is viable or capable of infection

Also, the virus has to be out there in the air in sufficient numbers to actually cause an infection. One would think that it only takes one single virus to infect someone, but generally it takes a much larger number. There seems to be a threshold for most viruses. The body dispenses of the first N viruses, then after that it gets harder, and eventually the system is overwhelmed. Maybe. The point is, virus experts will tell you that is has to be a large number for most viruses, and this is certainly true for SARS-CoV-2.

Truly aerosolized, viable, in sufficient numbers.

Finally, note that if SARS-CoV-2 was mainly airborne, we would know it by now. You can look at it, epidemiologically, and say, no, the main form of transmission is not airborne. That does not mean that there is not an airborne component, but it means that airborne is not the major way of spread. That has not changed.

What does the new research tell us?

Well, by the standards of peer reviewed scientific research, pretty much nothing, because that research is still in its infancy. But here is what happened. Several cases of infection have been reported that can be best explained by airborne infection. How many? So few that some would interpret that as potentially useless data. These may be cases that are simply misreported. Somebody licked someone else’s tongue and refuses to admit it. Does that really happen? Well, ask any expert on the epidemiology of sexually transmitted diseases about it. Of course it can. Most rare cases can be explained away or ignored.

But in this case, a large number of experts have settled on a provisional consensus: They see enough cases of possible airborne transmission of SARS-CoV-2 to ask the overarching institutional authorities like the CDC to seriously consider it and look into it. Yeah, that is it. Important, concerning, should shape policy modestly for now, requires more consideration. The smart money is on SARS-CoV-2 being transmittable via aerosol, though that will probably not be the main modality of transmission in most settings. That is my bet. Airborne transmission can happen, and will happen in some cases. More on that below. But, this is provisional.

What this does not mean.

This does not mean that there is a new mutation. Repeat: this is not a new mutation. This has been there all along, and the fact that it has not been obvious since the beginning means, as stated, this is not the new mode of transmission. This does not mean that the virus has changed. Probably.

This also has no impact on mask wearing. Airborne transmission will go right around the masks most people wear, but we already know that if airborne transmission is happening, it is not the main way the virus is spread. This is NOT AN ARGUMENT TO NOT WEAR MASKS so don’t go making that argument or you are a full-on jerk. Ignorant jerk. I know you won’t, but if you see that argument being made by others, that is what you are seeing. That argument is so stupid, you can expect Trump to make it soon.

What this might mean.

This is the important part of all this, worthy of careful consideration. Assume that normal near-distance non airborne transmission is the normal and most common form of transmission by a large margin. We assume that if people are kept a minimum of 6 feet away from each other (or 10 if you like) and do not share objects with their hands and faces, i.e, social distancing, that transmission will be minimized. This works for social gatherings, according to some, especially if masks are worn.

However, over longer term, while people are avoiding infecting each other by keeping their mucus to themselves, a low level background transmission via the air could be happening at a small level.

It would be rare. Say one hour of exposure within a single medium size room with modest air circulation has a one in a thousand chance of one infected person giving the disease to one other person in the room. (I am totally making up all these numbers, but just bear with me.)

But now, we take that room and put between zero and three infected people in it, and 30 target non infected people. But we put then in that room for 8 hours, and do that for 185 days. This configuration of people might sound familiar to you.

This is a classroom full of students social distancing. But wait, you say, if they are social distancing, they can’t fit 30 people in the room. But you would be wrong in some cases. Elementary schools with the pod system have four classes of 30 (including teachers) in the room. They will get their social distancing by spreading out into larger rooms in closed high schools or other places (gyms, etc.), so the main class of about 30 is still in one room. Maybe not. The point is, in the worst case scenario, we divide 1,000 by 8 (hours) then again by 185 (days) to get a baseline on transmission probability (though the math is slightly more complex than that) to arrive at this conclusion: Transmission within the classroom where there are one or two virus shedding individuals on any given day is nearly inevitable if there is a low probability of airborne transmission. Most classrooms may have zero infected people most of the time, but in a given school there would be several classes. In a given school system, maybe dozens and dozens.

If the air circulation does not remove the viruses, maybe they are being spread across the school. Students passing in halls, or any classes where the kids are reshuffled add to the dynamic, families with multiple kids (or both kids and staff) in the same school, etc. add to the dynamic.

You can do a similar calculation for restaurants and bars. Regular inside dining and bar hopping even with social distancing and mask wearing is probably not recommended if there is a low level of airborne transmission. More limitations on how retail shopping happens may be recommended. Certainly, unnecessary retail shopping maybe an unnecessary danger.

The final meaning of it all: When it comes to basic day to day life, under the current conditions of caution and distancing, this airborne problem would not have that much of an effect because it has to be rare. We know it is rare (if it is real) because if it was common we would see it. But, under school or large workplace reopening conditions, or reopening of indoor dining and shopping, etc., it may be a factor that causes two really bad problems.

1) More outbreaks, and some insidious ones. The school children, some getting very sick and maybe dying, others never becoming ill, passing the disease on to their families. Ignoring the airborne problem may involve asking our children to kill their grandparents, then live with that for the rest of their lives. You might get sick because you needed to shop for a new comic book or try out the headphones at the electronic store instead of ordering on line.

2) Not discussed anywhere else as far as I know, but I would think obvious: if we set up a situation where the rare airborne transmission has a better chance of actually transmitting the disease, we may also be setting up a positive selective environment for that. In other words, we may help make SARS-CoV-2 more airborne by giving it this chance. That is pure speculation on my part, but speculation based on some damn powerful theory (Darwinian evolution). It is not a chance I’d like to take.

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18 thoughts on “Understating Airborne Covid-19

  1. Please submit this to major news outlets! This is the only clear explanation of these topics I have seen.

  2. Does this explain why Fauci initially said masks are counterproductive? Now he says he was lying to protect supply for health care workers, but was it that mask makes you more likely to touch your face, and you are now breathing in virus that you have placed on your mask?

    1. Do you get paid to misrepresent things?

      Fauci’s comment that there was no need to wear a mask was in March. He said:

      I “Right now in the United States people should not be walking around with masks … You should think of healthcare providers who are needing them and the people who are ill.”

      That was the same message the Surgeon General, WHO, and the CDC were giving. In April

      (By early April) the Strategic National Stockpile had been depleted, and around the same time President Trump invoked the Defense Production Act to have manufacturing chains across the U.S. focus on making vital medical equipment such as ventilators and masks (although that didn’t go as advertised — nothing does with the asshole in chief).
      Fauci continued to say that they wanted to give as many masks as possible to front line workers and emergency personnel.(No change in message here — same as in March.)
      “We wanted to make sure that the people, namely the health care workers, who were brave enough to put themselves in a harm way, to take care of people who you know were infected with the coronavirus and the danger of them getting infected,” Fauci concluded.

      Later that message changed as more was learned about the spread of the virus, and wearing a mask was the message.

      Nothing underhanded, nothing hidden, right from the get go. Amazing how you conspiracy folks ignore that (and almost every other fact).

    2. The initial statements about masks were internally consistent. They were thought to work, but they were prioritizing their use.

      Now they are not prioritizing their use.

      But there was no sense of airborne transmission then, and THIS IMPORTANT POINT IS MADE IN THE POST: The possibility of airborne DOES NOT MEAN THAT THE MAJOR FORM OF TRANSMISION IS NO LONGER WHAT WE THOUGHT IT WAS. It still is, nothing has changed. Masks still help with that (and super masks would help with airborne transmission as well).

    3. That’s not all Fauci said. Nor the Surgeon General.

      “Later that message changed as more was learned about the spread of the virus, and wearing a mask was the message. ”
      Somehow Japan, South Korea, Taiwan, Singapore learned about the spread of the virus sooner than Fauci.

      Surgeon General:
      “STOP BUYING MASKS! They are NOT effective in preventing general public from catching coronavirus, but if healthcare providers can’t get them to care for sick patients, it puts them and our communities at risk!”
      He could have just left out that half sentence.

  3. New-ish, excellent, lab study on surgical mask effectiveness for reducing aerosol exhalation. Reduction was noteworthy (i.e., nontrivial) for the novel coronavirus, while confirming reduction is almost nonexistent for flu and rhinovirus, which already was known. Previously, the effectiveness of surgical masks for aerosols carrying the novel coronavirus was largely extrapolation from long knowledge of effectiveness for other infectious agents. Consilience with this new finding comes from previous knowledge that the novel coronavirus tends more than those other viruses, to be carried by larger droplets. Note that they did not study any face coverings of lesser quality than surgical masks, and they studied exhalations directly in front of the face rather than sampling the envelope around the person exhaling (so leakage from sides of mask were not adequately studied). That’s okay, this study was self-acknowledged to be narrow in purpose. https://www.nature.com/articles/s41591-020-0843-2

  4. No, MikeN, Fauci was not “lying.” It was, and still is, a cost-benefit tradeoff, which changes as additional information is learned. And yes, there are dangers from wearing face coverings, including (a) Most importantly, people being overconfident in them and so reducing other behaviors that are equally or MORE effective–distancing, not touching face, washing hands, staying far away from anyone who is coughing, sneezing, yelling, laughing loudly, singing, or has a higher than average probability of being infected or being susceptible to being infected. (b) Touching face including mask more often with hands, because people unaccustomed to and untrained in face coverings futz with them a lot–a LOT, as health care worker trainers can tell you. (c) Masks becoming repositories of virus, when not cleaned or discarded. Surgical and N95 masks cannot be cleaned, and are designed to be used only once.

  5. I agree with Tom Dayton – this is a clear explanation of the (somewhat semantic) nuances of airborne spread.

    There’s been a lot of pedantic discussion – and often mischaracterising – about aerosols and masks and whether the latter stops the former, especially amongst conservative anti-science types. The bottom line though is the evidence of numbers – to wit, the fact that New York, with its naïveté and its large and dense population, has very much flattened the curve where red (neck…) states have ignored the medical advice and are now experiencing millions of infections.

    It’s not rocket surgery. Wear a mask. Wash your hands. Keep your distance. And stop being a prat.

    1. Bernard J.

      And stop being a prat.

      You mean like Texas governor Greg Abbott:

      Texas has become one of the US’s new coronavirus hotspots, with new confirmed cases surging to around 14% of the country’s total, when measured by a seven-day average. Elective surgeries were paused this week as the state tries to free up hospital beds for increasing numbers of Covid-19 patients.


      Meanwhile in my neck of the woods (Southern Hampshire, England) following on from the opening of pubs three schools have had to close.

  6. MikeN accusing Fauci of lying when he supports a President who lies on a daily basis? Oh, the irony! How many lies has Trump made about Covid-19? Hundreds? Thousands? Everything from “It’ll be gone by April” to “We’ll open up by Easter” to “It’s 99 per cent harmless”, the narcissist-liar-in-chief makes up his nonsense as he goes along.

    MikeN, we all know here about your wretched views, but could you try and be a little less hypocritical?

  7. Re: Greg “The concept of airborne in thinking about pathogens is probably the single most misunderstood thing in epidemiology, not by epidemiologists but by regular people. ”

    I admit to thinking that the the term “airborne” meant something less precise that what Greg explained it to mean. As a geologist and as someone who spent some time in the military, the meanings with which I was familiar were quite different.

    I also taught science for a few decades and I think that most anyone who has taught science and tried to understand the misunderstandings and lack of understandings by students has considered the role of scientists in sewing confusion by developing scientific terms by simply redefining words used in ordinary language. Words such as theory, mineral, hardness, stress, force, work, power, speed, velocity, and acceleration are used by scientists in a much more rigorous (nuanced, nitpicky) way than most people use those words.

  8. There are too many instances by now where groups sing together, or shout together, or talk and laugh together in an enclosed space, especially without masks, and then a large percentage of the group gets ill. In those situations, it does not seem to be “rare” to breathe in enough of a SARS-CoV-2 dose to get infected.

    The fluid dynamics of real-life situations, where AC vents or breezes through open windows or people moving through a room create large air movements, seem to be contributing to spreading aerosols or larger droplets much further than earlier advice claimed. Those early claims were based on older studies into the physics of sneezing and coughing and such, done in still air. Real life air movement was explicitly excluded fro or licking doorknobsm those study conditions, and they were conducted with less sensitive instruments than the very recent pandemic studies have used.

    Who doesn’t know what it feels like to sit downwind of an AC vent, with tables of people between you and the vent? Who doesn’t know that you can smell the scents from those tables, and (when smoking was allowed) see the smoke from those tables drift across your own table – so why wouldn’t infectious bits also move with the air? Those normal observations should incline the scientifically minded to a) assume that disease transmission of a virus which attacks the upper respiratory system was very possible via air-carried bits, and b) any air-carried bits would be wafted fairly long distances by the air movement and c) work to get the funding to test whether or not those things which seemed likely were true or not.

    Why are churches and loud bars such major venues for spreading SARS-CoV-2 infection? Is it likely to be people licking other people? Is it likely to be people both spraying stuff from their orifices and breathing in what other people have sprayed? Depending on the venue, both may be important. Claiming that breathing in virus particles in floating gobbets of any size will only “rarely” spread infection just encourages the idiots who want to go to church and sing or go to loud bars and hang out and shout at each other over the loud music.

    And do we know yet how many virus particles you have to take in before your initial immune response is overwhelmed? No, we don’t. Some diseases take only a few, some take a lot more. We do know SARS-CoV-2 is very infectious. Is it likely that you need a big dose of virus particles to get infected?

    So yes, there are some good explanations of technical terms and good warnings about making assumptions based on studies done on other diseases in this post. But Greg then seems to make too many assumptions based on studies done on other diseases, and to be too dismissive of the mounting evidence for air-spread SARS-CoV-2 infections, in his conclusion that air-carried transmission is “rare”.

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