Tag Archives: Airborne

Understating Airborne Covid-19

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.

Ebola Will Not Become Airborne And Here Is Why

This discussion has been going on for some time, and a handful of recent events have prompted me to jump into it (beyond a simple comment or two). First, I saw a bunch of yammering among various biology teachers about this topic. Then Michael Osterholm wrote a well intentioned but seemingly deeply flawed opinion at the New York Times, then Dina Fine Maron wrote an excellent piece at Scientific American deconstructing Osterholm’s piece, then the latter two (and more) were summarized and expanded on in a post by Ann Reid at the NCSE.

Here, I will expand on this by applying first principles from evolutionary theory, organizing our thoughts in Tinbergenesque Terms.

There are four categories of reasons that Ebola won’t go airborne. I’m going farther out on a limb here than most others, who say things like “it is possible, but…” Imma say it just isn’t going to happen. Technically, over time, the Sus lineage of mammals (pigs) could give rise to a flying form, like what happened with some earlier lineage of mammal that gave rise to bats. So what I’m really saying is that Ebola will go airborne when pigs fly. Both are possible. But if that is what you really think of as “possible” instead of just “no, it won’t happen” than you may need to calibrate and stop buying those lottery tickets!

Here is why Ebola won’t go airborne.

First, diseases in general, including viruses, do change which species they infect sometimes, and they change in virulence and the exact effects on the host, but they really don’t change their mode of transmission. At the largest evolutionary scale there have been some novelties, obviously (or there would be no variation!). I am pretty sure many of the influenza viruses are not transmitted through the air, but the only ones we bother to name and study do, and are a subset of a larger group that transmits via water. I may have that wrong (going on old personal communications here) but if I am wrong that just crosses off Influenza as a virus that changed mode of transmission. Ebola is in a large group of viruses that are actually found in plants. Obviously, there was a change in transmission at the origin of Ebola. But really, this does not happen very often. If you can think of examples please tell me. (For a non virus example, Malaria is transmitted the same way all the time even if it changes (rarely) which species it affects or otherwise evolves like crazy to stay ahead of interventions.)

In short, we expect strong phylogenetic inertia in mode of transmission.

Second, there is no in place mechanism, probably. Ebola does not infect the tissues it would need to infect to make its way into a sneeze or cough. That would require a major change.

Third, developmentally, the first step in a virus’s life cycle is getting itself into a cell. Airborne viruses need to have a key that matches a lock on the outside of respiratory tissues. So Ebola not only lacks the means for getting out through a sneeze or cough, it also lacks the ability to do much if it did.

Fourth, it is not adaptive. Yes, a virus can mutate to do something stupid and maybe get a Darwin Award, but the chances are at least somewhat reduced. Ebola is very deadly in humans. Humans and the animal vectors that may stand between fruit bats (the likely wild host) and humans are not good hosts for Ebola. The chances of Ebola evolving to infect an unsuitable host are reduced.

Phylogenetically unlikely, mechanistically unlikely, ontogenetically unlikely, adaptively unlikely. Evolution is like baseball but slightly different. Four Tinbergen Strikes and you are out.

Now, the usual arguments in favor of Ebola doing the Hollywood thing rely on references to other viruses, like Influenza. Well, Influenza is way different from Ebola in its reproduction. It has a whole way of evolving that Ebola does not have. In fact, the differences is greater than, potentially (and rarely, but not never) the difference between evolution under sexual reproduction and evolution under simple replication. If two different Influenza strains infect the same cell, they can recombine (reassortment) to make an entirely novel never before seen Influenza. That is a very big deal and is thought to be the primary mechanism for the evolution of novel dangerous flu strains. Ebola does not do that. Ebola can’t do that.

Ebola does not do that. That thing Influenza does.

I said that twice. Now I’ll say it another way. Using Influenza evolution as a model for Ebola evolution is like using Primate Behavior as a model for Sea Slug Behavior. In other words, it does not fit.

Will Ebola go airborne? No.


I’m adding a bit more because some are still missing the point. This is an analogy that I think might be helpful

Cars fly, and airplanes drive around on the ground. Ebola can possibly be transmitted across space in a closed room from one person to another, and you can catch a flu by having someone with the flu bleed directly into your nose*.

But really, airplanes are vehicles designed to fly, they only drive around on the ground a little. They have wings, special engines, an overall shape and design that is adapted to flight. But really, cars only fly into the air now and then, and it is generally an accident.

An airborne virus replicates in high numbers in respiratory tissues, and causes the lysing (or some other process) of cells to allow itself out into mucous tissues. It is able to survive in mucous tissues, and then it is able to survive in aerosolized droplets. An aerosolized droplet is not a bit of bodily fluid cast into the air, it is not a drop of blood shed from a wound or bleeding eyeball, or a loogie. It is a bunch of liquid (mainly water) molecules coherent at a size sufficiently small that air currents are more important then gravity, so it becomes part of the atmosphere, and a virus may or may not be residing in it. Then, and airborne virus needs to have the external morphology that links up with a receptor site on respiratory cells in the individual subject to infection, and then, it reproduces mainly in that tissue.

Ebola is none of these things, except possibly one. Ebola is known to survive in mucous tissue for some time after it has left an infected individual. This is not the same as surviving in an aerosolized droplet, but it indicates the possibility. But to go back to the car-airplane analogy, that is a bit like saying that some cars fly farther when they leave the road during an accident.

The distinction is very important. Jane, commenter below, has oddly implied that I’m not taking Ebola seriously. I would like to point out that I may have been the only person to complain about and argue against the trope that Ebola is not so bad because it is not Malaria. I may also be one of the few bloggers writing about Ebola who has lived in Ebola country, doing health care work, and who has actually worked on the problem of the natural reservoir and contributed to it. I am also one of the few people writing now who has pointed out that even though most people with Ebola are in a few African countries, where this needs to be taken very seriously, that it is also true that those communities, in West Africa, are global. This is how my neighbor, Patrick, managed to die of Ebola. He was an American who also worked for the Liberian government, and was in Liberia taking care of his sister, who died of the disease. His wife and family are here, in my town. Ebola affects communities that are not separate from those who have the privilege of being able to muse about it. And here is where the distinction becomes multi-dimensional. All the talk about airborne transmission is not scientifically grounded, and it is a distraction. But saying that it will not become airborne is not saying that it is not a horrible disease that is highly infectious and has pandemic potential. This, the nuances of the epidemiology of Ebola, isn’t really that complex, but sadly, it is a bit too complex to be well managed by the press and others talking about it, in many cases.

And, the distinction is huge. Conflating the very small number of possible infections “across the room” (which are speculative but possible) in prior outbreaks (which, Jane, were not in East Africa) with an airborne mode of transmission is like working out transportation policy for the US but mixing up the part about how cars don’t fly and airplanes do. I really think Ebola is not going to become airborne. But if it was airborne, the whole ballgame would be very very different. That, however, does not mean that Ebola is not a very serious thing that needs to be addressed. Also, the utter failure to address this by the systems in place tells us that we as a society/species/collection of governments are unable to address a serious public health crisis even if we were under the impression that we were. Trading in misinformation and badly conceived ideas of what is happening or what could happen sets us back, it does not move us forward.

More on Ebola:

*Actually, this may not be true, to my knowledge no one has considered this, certainly not tried it!