Throughout the pandemic, public health officials have offered guidance on how to slow the spread of COVID-19. Since March 2020, we’ve learned how to properly wash our hands. We’ve practiced physical distancing. And wearing face masks has become a regular part of our daily routine. All these interventions are designed to slow the spread of COVID-19 based on everything we know about how respiratory diseases typically move from one person to the next.
But with everything new we’ve learned about COVID-19, do you ever wonder what we actually know about how it spreads?
Jose-Luis Jimenez (@jljcolorado), a professor and aerosol scientist at the University of Colorado, Boulder, talked with us about the science of COVID-19 transmission. You may have heard of him or some of his colleagues when more than 200 scientists published an open letter to the WHO supporting the claim that COVID-19 is an airborne disease. Or you may have seen his Twitter thread on ten scientific reasons in support of airborne transmission of COVID. Our conversation was a fascinating glimpse into the history and process of science, with important implications for ending the pandemic.
Have a question you've been wondering about? Send an email or voice recording to the podcast team to tell us what you'd like to hear in future episodes.
Perry Roth-Johnson (00:06):
Hello! This is Ever Wonder? from the California Science Center. I'm Perry Roth-Johnson. Throughout the pandemic, public health officials have offered guidance on how to slow the spread of COVID-19. Since March 2020, we've learned how to properly wash our hands. We've practiced physical distancing. And wearing face masks has become a regular part of our daily routine. All of these interventions are designed to slow the spread of COVID-19 based on everything we know about how respiratory diseases typically move from one person to the next. But with everything new we've learned about COVID-19, do you ever wonder what we actually know about how it spreads?
Perry Roth-Johnson (00:46):
We invited Jose-Luis Jimenez on the show to talk about the science of COVID-19 transmission. He's a professor and aerosol scientist at the University of Colorado, Boulder. You may have heard of him or some of his colleagues when more than 200 scientists published an open letter to the WHO supporting the claim that COVID-19 is an airborne disease. Now, in this conversation, you'll hear us talk a lot about aerosols versus droplets, what kinds of evidence helps us understand how diseases like COVID-19 spread, and why it's been so hard for some medical and public health experts to let go of the so-called "droplet dogma." It's a fascinating glimpse into the history and process of science with really important implications for ending the pandemic. Take a listen. Jose-Luis Jimenez, you are a professor of chemistry and an aerosol scientist at the University of Colorado, Boulder. Jose, welcome to the show!
Jose-Luis Jimenez (01:44):
Thanks for having me.
Perry Roth-Johnson (01:45):
Yeah! And Devin Waller, co-host of the show, is also here with us. Hi Devin!
Devin Waller (01:49):
Hey Perry, great to be here. And hi Jose, thanks so much for joining us.
Perry Roth-Johnson (01:52):
So, Jose, one of the reasons we're so excited to talk to you is because you recently published an article in The Lancet titled, "Ten Scientific Reasons in Support of Airborne Transmission of SARS-CoV-2." Now I bet many of our listeners already know this, but just to be clear, SARS-CoV-2 is the virus that causes COVID-19. Um, but throughout the pandemic, you've also been active on Twitter trying to communicate some of the science around COVID, uh, before we get into all of that though, can we just start with some basics? Like why do we study disease transmission in the first place? Why is that useful for public health protocols for controlling the spread of a disease?
Jose-Luis Jimenez (02:29):
Well, it is very important to know how a given disease is transmitted in order to be able to protect ourselves from it. Like if we know a given disease is transmitted through mosquitoes, like malaria, or through sexual contact, like HIV, or through airborne transmission, like COVID, then we need to do very different things to protect ourselves. So that's why many scientists study what are all the possible ways in which each disease is transmitted.
Perry Roth-Johnson (02:56):
And how does your expertise as a professor of chemistry and an expert in aerosols, you know, help us understand disease transmission better, you know, specifically with COVID-19?
Jose-Luis Jimenez (03:06):
There are some diseases that have been known for decades like tuberculosis or chickenpox or measles that are transmitted through the air, through aerosols. This means that there are little bits of saliva, respiratory fluid that leave a person, and they float through the air and other people can inhale them. And this is what we call airborne transmission. And, I started investigating with many other scientists, whether this was playing a factor, was playing a role in the transmission of COVID-19. And at the beginning of the pandemic, we were told that it was not. You know, that the WHO was certain this was not important. Um, but throughout the pandemic, really, there has been a lot of research that has shown that that was incorrect. And really it's probably the main way in which COVID is transmitted.
Perry Roth-Johnson (03:53):
And like what are you studying, you know, before the pandemic? What, what kinds of things were you studying that seemed to kind of translate, um, between your earlier research and COVID-19 during the pandemic?
Jose-Luis Jimenez (04:04):
Well before COVID-19, um, I studied aerosols. I studied aerosols for 20 years for different applications. So we have studied how aerosols behave indoors and also outdoors in terms of pollution. The, you know, the aerosol pollution or particulate matter as it's also called, is the pollutant that kills the most people worldwide, about 7 million people a year.
Perry Roth-Johnson (04:27):
Jose-Luis Jimenez (04:27):
And it also plays a role on climate change. So for all these reasons we have studied, you know, how do aerosols come into the air? How do they behave once they're in the air? How do they come out of the air? How do people breathe them in? How to build mathematical models to explain, or to quantify, all of those different things? So all of these skills prepared me well to understand COVID transmission.
Devin Waller (04:50):
Jose, a few months ago, we opened an exhibit at the Science Center. It's called All in This Together. It's about COVID-19 and how science helps us understand how our individual actions affect broader communities. And in the exhibit, there's a video called The Air You Share, where we show how far the air that comes out of our mouths and noses that we breathe, um, spreads. And we've seen you tweeting about dispelling the "droplet dogma". Can you first talk about the difference between what is a droplet and what is an aerosol, and how do they differ?
Jose-Luis Jimenez (05:26):
So, first of all, these terms can be used differently by different scientific disciplines. So for atmospheric sciences, everything is a droplet if it's wet. But in the field of disease transmission they have specific terminology in which a droplet is something that falls to the ground quickly. It's a projectile. So, you know, if you are coughing or if you're sneezing, or sometimes if you are, if you're shouting or you are speaking in a strong voice, you can see visible little bits of fluid of saliva that leave person and fly through the air. But they behave like a projectile and then they fall to the ground. And that can result in infection. For example, if someone coughs on your face and some of those droplets they dispel hit the inside of your eye or the inside of your nostrils or the inside of your mouth that can lead to infection. So those are the droplets. The aerosols are the same, literally the same material, also saliva or respiratory fluid, but they are much smaller. And because they are much smaller, they no longer behave like a projectile or fall to the ground. But instead, that approximation is that they behave like cigarette smoke. Cigarette smoke leaves a person and it doesn't fall to the ground. At the beginning, it comes out with some speed and then by friction, it stops. And then it follows air currents. It may go up, it may go down, you may go sideways. It may stay in the room if it's poorly ventilated. And you get infected by aerosols, not because they impact you, but because you breathe them in.
Devin Waller (06:53):
How do we know how COVID-19 spreads? So in April of 2021, you had this Twitter thread about a Lancet article that you coauthored that lists 10 pieces of evidence to support airborne transmission of COVID-19. So can you give us maybe like the top two to three pieces of evidence that you listed in the article and can you break them down a bit?
Jose-Luis Jimenez (07:16):
Yeah, so say in principle a virus like this could go in three ways. It could go through surfaces, someone, um, coughs on their hand, and then you touch their hand, you touch, and then you touch your eye. But now we know that that's very inefficient. So the CDC told us that's a chance in 10,000, or something like that. So then really we are left with what we discussed before, the droplets or the aerosols. So it's either the projectiles that leave a person that can impact someone else in the face or otherwise fall to the ground, all the other sorts of float around like cigarette smoke, and you can breathe them in. So we'll have to distinguish between these two possibilities, right? And we can look at many different types of evidence. So I think probably the clearest one is that for this disease is there is a huge difference in transmission, indoors versus outdoors. There are many studies and you're at least 20 times less likely to get infected outdoors, maybe as much as a thousand times more difficult to get infected outdoors.
Devin Waller (08:17):
Jose-Luis Jimenez (08:17):
And, you can imagine if you're talking to someone or someone is coughing and they are producing these projectiles, these projectiles are going to do the same, indoors or outdoors. The gravity is the same. On the other hand, aerosols, if you're smoking, you know if someone is smoking outdoors, the smoke will rise and be dissipated much, much faster. Well, if you are indoors the walls and the ceiling are going to stop that smoke and give you many, many chances to breathe it in, especially if that place is not ventilated, which means that the air is trapped basically. So if it was the projectiles, we will expect indoor and outdoor to be the same. If you, if it's dominated by the aerosols, we expect indoor to be much easier for transmission, which is what we see. Then there is also superspreading events, which these are situations like the choir case that we studied earlier in 2020, in which one person, for example, infected 52 in a choir rehearsal. And in that case, we know we talked to the choir and we got information from them in great detail. We know what they did, and we know that they didn't touch any common surfaces because they already knew about COVID, although they didn't know it was in the community yet. And we know that they didn't spend enough time, um, talking to each other or anything like that for these droplets to be the means of transmission. So only airborne transmission was possible, right, through these aerosols. And there are many, many superspreading events, and there is not a single one that has been published that has been connected to these large droplets. Every single one points towards the transmission through the air. And these superspreading events are very important for the pandemic because many people who have COVID don't give COVID to anybody, but then 10% of the people give it to 80% of the newly infected, right.
Perry Roth-Johnson (10:08):
Jose-Luis Jimenez (10:08):
So, this superspreading is very important. And to mention another one of the different types, there are experiments with animals. So we know, for example, with hamsters or with ferrets, they can get COVID and they can give COVID to other animals. And there are experiments that have been done with hamsters and ferrets in which their only communication is through the air, right. They are not touching. They cannot, if they cough, they cannot reach each other and they get other animals infected efficiently. Right. So in animal models, we see that the infection through the air happens efficiently.
Perry Roth-Johnson (10:43):
In July 2020, I saw an article in the L.A. Times about how you and 200 plus scientists signed an open letter to the CDC and the WHO challenging the official view of how COVID spreads. What was in your mind, what was going through your mind when you signed that letter?
Jose-Luis Jimenez (10:59):
That letter appeared on July 6th, but we had been working on it for a couple of months. Um, but really the catalyst is on the 20th of March of 2020, WHO comes out with an announcement, a tweet, on Facebook where they say in big capital letters, "FACT: [COVID-19] is NOT airborne". The disease COVID-19 doesn't go through the air. It goes through these large droplets and on surfaces. And to say that it is airborne is misinformation, and we have to all help WHO fight this misinformation. So now we thought this was not consistent with the evidence that was accumulating, that there was airborne transmission such as the case of the choir that we were already investigating at the time. So Lidia Morawska, who's an Australian scientist, put together a group of scientists, and they contacted me and many others from different countries and from different disciplines, to talk to the WHO together and WHO responded very quickly. And we had a conference call on the 3rd of April, just a few days later. And we talked to Maria Van Kerkhove, who's the person managing the pandemic for WHO, as well as the Infection Prevention and Control Committee, who are the scientific experts that advise WHO and that had decided that this disease was not airborne. And we had a good conversation, but it was clear that to them, the idea that the disease was airborne was crazy. They were completely stuck on this thing we call the "droplet dogma". They were completely convinced it was a droplet disease and that airborne transmission was negligible. And we were not going to make a lot of progress. We exchanged some more letters, one of which I coordinated. But it was clear they were not. We were not going to make progress that way. And at the same time, the evidence was accumulating. So we decided to write a letter. There was also a scientific paper. And so we wrote that between the group of 36, but then we realized that because this was because of WHO's announcements, the press was viewing us very skeptically, right? If WHO says in capital letters, it is a fact, there's no airborne disease. This is misinformation. And now there is Jose The Scientist saying it's airborne. You know, I sounded like some, some crazy person that's saying that it's 5G that transmits COVID.
Devin Waller (13:19):
Jose-Luis Jimenez (13:20):
So, then we decided to ask for more colleagues for signatures. And we asked, I mean, we decided to only ask high level colleagues. And basically everyone we asked, agreed to sign and they brought a couple of friends. So very quickly we got to 239. And really then things changed because I think it became clear to the WHO and it became clear to the press that this was a mainstream scientific debate. That there was a major fraction of the scientific community that did think the disease was airborne. And this couldn't just be neglected as misinformation.
Perry Roth-Johnson (13:51):
Right. Right. That makes sense. Because so many times the root of other pieces of like real misinformation, or like there's a single study, or there's like a pre-print that hasn't been peer reviewed and somebody latches onto an amplifies it. But your strategy of gathering 239 respected scientists who, you know, not only wrote this open letter, but you said it was a peer reviewed article as well, that was published from the letter, you know, gave you guys firmer footing to stand on. And it became more mainstream, which is why I probably saw it in the L.A. Times.
Jose-Luis Jimenez (14:25):
Devin Waller (14:25):
Now, if we fast forward to a few months ago, this is now late April and early May of 2021. That's when we see the WHO and the CDC update their COVID web pages, adding that now the virus can spread via aerosols. So they've now admitted this. Why did droplets get stuck in the minds of medical and public health officials for so long?
Jose-Luis Jimenez (14:49):
That's a very good question. I think, what I think the answer is, is that there are several reasons, but one very important reason is history. So, you know, throughout the history of humankind, since Hippocrates, who was a Greek scientist, I guess. Um, he said, you know, when a lot of people get sick, we have to suspect the air because that is where we have the most in common and really throughout most of human history, there were people who thought that diseases were transmitted through the air. But then, um, germ theory, you know, with Pasteur and Koch, is developed the late 1800s. And then we know, okay, infectious diseases necessitate some pathogen to go from one person to the other. And then people start investigating, okay, for cholera, for malaria, for typhoid fever, how does the pathogen go from one person to the other? And they realized that this is different for different diseases. And so then in 1910, there is a public health expert named Charles Chapin, uh, who later became the President of the American Public Health Association. And in 1910, he writes a book where he compiles all the evidence that has accumulated since germ theory about different diseases or how they are transmitted. And he has a pet theory. He has a theory that, uh, that he calls contact infection that you get infected if you touch someone or if you are very close to them, right. And he really wants to promote, he thinks that that's really the main mode of transmission for respiratory diseases, and really wants to promote that. And he knew from observations that, for example, if you keep more distance, there is less infection. For example, in the Crimean War, they realize that if they put the beds of the soldiers farther apart, there was less infection. And there was information like that, right? So, distance helps, but he doesn't know why. But he hypothesizes that it is because of these droplet projectiles, right. If the projectiles fall to the ground, if you keep more distance, then you don't get infected. So this is droplet infection. As he calls it, it's a sprayborne infection. And then he's also aware that you can go through the air in smaller aerosols but at the time they couldn't measure them. And he says in the book that if people think they are getting infected through the air, they don't pay attention to him and contact infection, because you are going to get infected through the air anyways. It's unavoidable, right? So they don't wash their hands. They're going to keep the distance. So then he says, I'm going to say, since we don't have evidence that airborne infection is almost impossible and whoever wants to prove airborne infection really has to prove it beyond any reasonable doubt, like in a criminal trial.
Devin Waller (17:26):
Jose-Luis Jimenez (17:26):
And, um, he proposes this. And the problem is that he's too successful. This becomes the dogma. And this is the beginning of the "droplet dogma". And since 1910, until COVID in 2020, this has been the dominant view in epidemiology and infectious diseases. And, um, as we say in Spanish, everyone who is holding the pan by the handle—everyone in command, the WHO, the CDC, the Ministries of Health in public health—has been implicated for a hundred years on this dogma.
Perry Roth-Johnson (17:57):
Jose-Luis Jimenez (17:57):
Diseases don't go through the air, this is very rare. There is very few diseases that do that, like tuberculosis or whatever, but, but most diseases don't do that. And if a disease is transmitted well when you are close to someone, but then when you keep your distance it's transmitted much less, well, that means it's a droplet disease, right? And this is an error. But, but that has been just a dogma. And the CDC, when the CDC was created, the person who was in charge of the epidemiology branch of the CDC and who was the head for 20 years, Alexander Langmuir, was a complete Chapinista. He absolutely believed that. He said later that Charles Chapin was the greatest American epidemiologist. And they were actually misinterpreting the studies in a way that's obvious for an aerosol scientist, but that was not obvious to them. But they were misinterpreting the studies they were doing so that they didn't see the airborne transmission that was in front of them. So I think this is the main reason you enter. So we started the COVID-19 pandemic. If we were in a situation in which the public health and infectious diseases community didn't have a preconceived notion of how it was transmitted, you can imagine naturally some people will think it’s through the air, some people will think it's droplets, some people will think—and there will be a more robust debate, but we enter COVID-19 and there is a monolithic belief almost complete that, "No, no, no! Diseases don't go through the air. This is a droplet disease." And everybody in a position of power thinks that way. And the people who are challenging that, are people like myself, who are outsiders and who are not, who are viewed as kind of ignorant, you know, in this, and WHO said as much, you know. They, they described us as ignorant on disease transmission. The problem is that now for 110 years, this community has thought that aerosols were not important. Now you don't learn study something that's not important. And aerosols are actually pretty complicated. So these people, they not going to lead about aerosols. And when they open their mouth, they make a lot of errors. And, um, and this is, this is a big problem, but, but that's the situation we find ourselves in. And, I realized that, um, I mean, sometimes people hear this and I have to say, even at the beginning, when, when someone was telling me about Chapin and before I was like, "What? This sounds so weird." But I've investigated it. And I've learned about it. And it is what it is, you know. The world is as it is and not as we would like it to be.
Perry Roth-Johnson (20:35):
Well, we're so glad that you were able to join us and share your story and your expertise, Jose. Um, what else are there any other stories you'd like to share? Uh, what else do you want people to know?
Jose-Luis Jimenez (20:47):
I mean, there is one thing that I learned, you know, when I was investigating this "droplet dogma", and then seeing that people were, so convinced. You know, these epidemiologists and the experts are convinced that the droplets were so important. So I thought, they must have, you know, amazing evidence. And then I read a paper that said that they had done a literature search on large droplet transmission. And they had found that large droplet transmission had never been demonstrated directly for any disease in the entire history of medicine.
Devin Waller (21:22):
Jose-Luis Jimenez (21:22):
So, I almost fell from my chair. WHO tells us they are sure this is a droplet disease, but droplet transmission has never been demonstrated directly. It has been assumed based on indirect evidence. And, then I started tweeting this and asking people, if you have a scientific article that proves otherwise, please send it to me. And I'm still waiting. Nobody has sent me any articles that proves otherwise. I mean, this is also something quite shocking. And similarly with surfaces, you know, WHO early on told us that the surfaces were very dangerous. And, you know, early on in the pandemic, we were already scared. And we didn't know about this virus. They said, surfaces are what transmits it. So myself, like everybody started disinfecting the groceries and quarantining the mail and all these things.
Perry Roth-Johnson (22:12):
Yeah. Me too.
Jose-Luis Jimenez (22:12):
And, um, you know, we did that for months. I mean, it took me months to convince my wife that maybe this wasn't needed. You know, but they did that without having evidence. It was again, based on a committee, having a committee of experts that was biased that had six people who were experts on hand-washing and who believed that this was an important model of transmission, but the evidence really has not backed it up. But that has really created, you know, a really bad pattern because whatever you learn at the beginning is hard to give up. And in many, many countries, you still see people, you know, they don't open the windows, they don't wear good masks, but they disinfect the surfaces religiously, you know, and even you see in many countries, they go around the streets, disinfecting the streets, which it's polluting, and it's an absolute and complete waste of time. The sun is already good at disinfecting outdoors and outdoors there is almost no transmission anyway. So, that just serves no purpose, right? And then there are other things like, um, plexiglass barriers, right. When we are told that basically these little projectiles of spit, that's how the disease is going to get you. Then we put barriers around everybody so that we don't get sick, right? But then it turns out that these barriers, I mean, when they are between you and someone else in a cashier, in a teller situation, they're useful because they break the flow of air. Like they will stop the smoke and they have to travel more to reach the other person. But when they're lateral, not only do they not help, but actually double transmission. There is a study in Science, the most prestigious scientific journal, saying they actually double transmission. And it makes sense because all these barriers trap the air, they may ventilation more difficult. So if the virus is there, because someone is exhaling it, these barriers are trapping it and giving people more chances to breathe it in, yet in the us and worldwide, we will spend billions of dollars in these barriers. You know? So, I mean, at the end, what's frustrating is that the things that we need to do to limit airborne transmission are not so difficult. We need to explain to people that it is like a smoke, then people really can get it. We need to do things outdoors. We need to open the windows. We need to wear good masks that are very good fit to our face. And we need to filter the air. Just like, we wear a mask, we can have a fan with a filter and we need to filter the air. Many of these things are free or they're certainly not more costly than disinfectants and plexiglass barriers and things like that. Yet they are not being done systematically. And for that reason, the pandemic keeps being very difficult to control.
Devin Waller (24:47):
Jose, as we wrap up, um, science communication is very important. And we noticed that you often tweet in both Spanish and English and it's, it's so important that we reach as many people as possible. And social media is a great communication tool. Um, what do you hope to accomplish by tweeting bilingually?
Jose-Luis Jimenez (25:04):
I am originally from Spain. So, you know, I was trying to get the word out, as I said, to people onto other scientists. So naturally I started tweeting in both languages. Then I realized that for example, in this group of the 36 scientists that we were, many of us were very active in trying to communicate around transmission. I was the only one who was doing so in Spanish while there were many people who were doing a very good job in English. So then I felt it was, I should take it on my shoulders to communicate in Spanish and really I have done an enormous number of interviews in Spain and in Latin America, I think in almost every country.
Devin Waller (25:42):
Jose, thank you! This has been a very interesting and important conversation. Where can people follow you online and find your work?
Jose-Luis Jimenez (25:50):
I'm on Twitter. My handle is @jljcolorado. And that's, uh, if you go from there, you can find everything else.
Perry Roth-Johnson (26:01):
Perfect! It's been wonderful talking to you, Jose. Thanks for spending the time with us and joining us on the show.
Jose-Luis Jimenez (26:07):
Thank you very much for having me.
Perry Roth-Johnson (26:09):
That's our show, and thanks for listening! Until next time, keep wondering. Ever Wonder? from the California Science Center is produced by me, Perry Roth-Johnson, along with Devin Waller. Liz Roth-Johnson is our editor. Theme music provided by Michael Nickolas and Pond5. We'll drop new episodes every other Wednesday. If you're a fan of the show, be sure to subscribe and leave us a rating or review on Apple Podcasts—it really helps other people discover our show. Have a question you've been wondering about? Send an email or voice recording to firstname.lastname@example.org, to tell us what you'd like to hear in future episodes.