In a letter published in The Lancet, a group of scholars argue, with an extensive review of the available evidence, that the primary mode of transmission from human to human of the virus responsible for Covid-19 is via aerosols, not through larger particles called droplets or through fomites (transfer from surfaces). Here are three of their ten reasons why:
Third, asymptomatic or presymptomatic transmission of SARS-CoV-2 from people who are not coughing or sneezing is likely to account for at least a third, and perhaps up to 59%, of all transmission globally and is a key way SARS-CoV-2 has spread around the world, supportive of a predominantly airborne mode of transmission. Direct measurements show that speaking produces thousands of aerosol particles and few large droplets, which supports the airborne route.
Fourth, transmission of SARS-CoV-2 is higher indoors than outdoors and is substantially reduced by indoor ventilation. Both observations support a predominantly airborne route of transmission.
Fifth, nosocomial infections have been documented in health-care organisations, where there have been strict contact-and-droplet precautions and use of personal protective equipment (PPE) designed to protect against droplet but not aerosol exposure.
The letter concludes with a plea by the authors for public health officials to finally embrace this reality: “The public health community should act accordingly and without further delay.”
I can’t believe we’re actually still arguing about this. One of the authors, Jose-Luis Jimenez, wrote this seminal Time magazine piece that provided the smoke analogy that is the mental model I’ve been using to think about potential risks during the pandemic.
When it comes to COVID-19, the evidence overwhelmingly supports aerosol transmission, and there are no strong arguments against it. For example, contact tracing has found that much COVID-19 transmission occurs in close proximity, but that many people who share the same home with an infected person do not get the disease. To understand why, it is useful to use cigarette or vaping smoke (which is also an aerosol) as an analog. Imagine sharing a home with a smoker: if you stood close to the smoker while talking, you would inhale a great deal of smoke. Replace the smoke with virus-containing aerosols, which behave very similarly, and the impact is similar: the closer you are to someone releasing virus-carrying aerosols, the more likely you are to breathe in larger amounts of virus. We know from detailed, rigorous studies that when individuals talk in close proximity, aerosols dominate transmission and droplets are nearly negligible.
Another of the authors, Zeynep Tufekci, has been arguing the case for aerosols (and masks & overdispersion) since early in the pandemic, and she succinctly explained in a Twitter thread how predominantly aerosol transmission fits with the mitigation methods that have really worked around the world:
Airborne transmission unites three things crucial to recognize for effective COVID-19 mitigation: transmission without symptoms (thus aerosols), clusters driving the epidemic (also aerosols) and masks/ventilation indoors being key (hey, also aerosols). This framework is coherent.
Her whole thread is worth a read — like this bit about how other respiratory pathogens are likely spread by aerosols and not droplets (as commonly believed):
Fascinatingly, you search the scientific record high and low, but there really is little to no direct evidence for gravity-sprayed droplets being predominant mode of transmission for respiratory illnesses outside of coughing/sneezing. It’s many… assumptions. Like a tradition.
If any good comes out of the pandemic at all, a better and more useful scientific understanding of how respiratory pathogens are transmitted would be a good start.
Update: One of the authors, Trisha Greenhalgh, responds succinctly to criticisms of the paper in this Twitter thread.
Criticism 1: “The paper is just opinion, and several authors aren’t even doctors.”
Response: No. It’s well-researched scholarly argument, produced by an interdisciplinary team of 6 professors including 3 docs, 2 aerosol scientists and 1 social scientist.
A spin studio (aka an indoor gym with stationary bikes) in Hamilton, Ontario is dealing with an outbreak of Covid-19 stemming from one asymptomatic patron that has resulted in 69 positive cases so far, even though the studio “followed the rules”. From the CNN report:
SPINCO, in Hamilton, Ontario, just reopened in July and had all of the right protocols in place, including screening of staff and attendees, tracking all those in attendance at each class, masking before and after classes, laundering towels and cleaning the rooms within 30 minutes of a complete class, said Dr. Elizabeth Richardson, Hamilton’s medical officer of health, in a statement.
As the Washington Post reports, patrons were allowed to take their masks off while exercising:
Although Hamilton requires masks to be worn in most public settings, the law includes an exemption for anyone “actively engaged in an athletic or fitness activity.” In keeping with that policy, the studio, SPINCO, allowed riders to remove their masks once clipped into their bikes, and told them to cover up again before dismounting.
The problem here is that while the studio may have followed the rules, they were not the right rules. This outbreak appears to be another clear-cut instance of Covid-19 spread by aerosols. A group of people indoors, without masks, breathing heavily, over long periods of time in what I’m guessing is not a properly ventilated room — this is exactly the sort of thing that has been shown over and over again to be problematic.1 The science is there, but governments and public health agencies have not caught up with this yet. If you take the transmission by aerosols into account, the rules for gyms (and bars and restaurants) being open is that they should probably not be open at all — or if they are, they should be well-ventilated and the wearing of masks should be mandatory at all times.2 (via @DrEricDing)
In an article that The Atlantic classifies as “politics” rather than “science” or “medicine”, Olga Khazan explores why, more than 8 months into the pandemic, Americans still have little idea about the safety of gathering with others indoors.
For months now, Americans have been told that if we want to socialize, the safest way to do it is outdoors, the better to disperse the droplets that spew from our mouths whenever we do anything but silently purchase grapefruit. But in many parts of the country, this is the last month that the weather will allow people to spend more than a few minutes outside comfortably. And next month, America will celebrate a holiday that is marked by being inside together and eating while talking loudly to old people.
In a nutshell, the lack of federal support/guidance/action is the main reason why people are still so confused about what safety measures to take to reduce their Covid risk:
Still, Ranney says, this [Covid risk] app is the kind of thing the federal government really should have developed by now. It’s odd that in a wealthy, industrialized country, a random researcher is the one designing a tool to keep citizens safe from public-health threats, using data she scraped from a newspaper.
One thing that Khazan doesn’t really get into is the whole aerosols thing, which in my mind is something that most people are still not familiar with, many local & state governments are not taking into account w/r/t recommended safety measures, and requires different risk guidance about the safety of the indoors than if we were just dealing with fomites & droplets. Again, from the excellent Time magazine piece by aerosol chemist Jose-Luis Jimenez:
When it comes to COVID-19, the evidence overwhelmingly supports aerosol transmission, and there are no strong arguments against it. For example, contact tracing has found that much COVID-19 transmission occurs in close proximity, but that many people who share the same home with an infected person do not get the disease. To understand why, it is useful to use cigarette or vaping smoke (which is also an aerosol) as an analog. Imagine sharing a home with a smoker: if you stood close to the smoker while talking, you would inhale a great deal of smoke. Replace the smoke with virus-containing aerosols, which behave very similarly, and the impact is similar: the closer you are to someone releasing virus-carrying aerosols, the more likely you are to breathe in larger amounts of virus. We know from detailed, rigorous studies that when individuals talk in close proximity, aerosols dominate transmission and droplets are nearly negligible.
If you are standing on the other side of the room, you would inhale significantly less smoke. But in a poorly ventilated room, the smoke will accumulate, and people in the room may end up inhaling a lot of smoke over time. Talking, and especially singing and shouting increase aerosol exhalation by factors of 10 and 50, respectively. Indeed, we are finding that outbreaks often occur when people gather in crowded, insufficiently ventilated indoor spaces, such as singing at karaoke parties, cheering at clubs, having conversations in bars, and exercising in gyms. Superspreading events, where one person infects many, occur almost exclusively in indoor locations and are driving the pandemic. These observations are easily explained by aerosols, and are very difficult or impossible to explain by droplets or fomites.
The science is there — it’s the lack of connection between scientists, public health experts & officials, and the government that continues to be a problem.
A group of scientists who believe that WHO and the CDC are being too slow in acknowledging the role of aerosol transmission in spreading Covid-19 have written up a Google Doc of advice for the public: FAQs on Protecting Yourself from COVID-19 Aerosol Transmission.
The goal of these FAQs is to provide information to the general public in an efficient manner about how to prevent aerosol transmission of COVID-19, with the hope that this will allow more informed decision making by individuals or organizations. All of this information has been posted in Twitter and other forums, but can be difficult to find. Having multiple experts working together, and having the ability to update this information also improves its quality. These FAQs represent our best understanding at this time, and should always be similar or more stringent than information provided by CDC, WHO, and most regional & local health authorities. If your authority has a more stringent guideline than discussed here, follow that more stringent guideline.
The group was organized by chemist Jose-Luis Jimenez, who has been studying aerosols for 20 years. You may remember Jimenez from his excellent piece in Time magazine, where he used the analogy of smoke to explain aerosol transmission. Here’s a snippet from the FAQ, highlighting something I’ve been concerned about lately: people wearing face shields instead of masks and employees in stores not wearing masks behind plexiglass shields:
7.13. Are face shields and masks interchangeable?
No, face shields do not offer much protection against aerosols (also see this video), while masks do. Face shields are good for blocking ballistic droplets released by the wearer or that might fly into the wearer’s face when close to others. Face shields are considered a supplement to masks for partial eye protection (but less useful than closed glasses, as discussed above), but not a substitute for them.
7.14. Are plexiglass barriers helpful?
Plexiglass barriers are generally useful to avoid direct droplet infection and direct aerosol transmission whenever people are in close proximity and distance cannot be kept. Therefore, it is recommended to use them as a direct transmission suppression tool at such places, such as a supermarket checkout.
However, as aerosols follow the air movements indoors, the protective effects of the plexiglas barriers against aerosols will be limited. Plexiglas barriers alone are not a sufficient approach to protect against aerosol transmission. Their installation alone cannot protect against indoor aerosol transmission and should not be regarded as safe and sufficient protection.
MIT Technology Review’s Charlotte Jee interviewed Jimenez about the FAQ document.
We update the document all the time. We’re effectively having to be a little WHO or CDC. We’re saying the things that they should be saying. This is frustrating, but it’s the situation we find ourselves in. These organizations have been flat-out refusing to consider if aerosol transmission is important, which leaves people unprotected. So we feel it’s our duty to communicate directly with the public.
Right now, in my opinion as someone who has done a ton of reading about Covid-19, the most best accessible information on how individuals and societies can protect themselves and others during the pandemic (and why) is available in Jimenez’s Time article, Aaron Carroll’s NY Times piece about how to think about risk management, Zeynep Tufekci’s piece in the Atlantic about dispersion and superspreading, and now this Google Doc by Jimenez et al.
Chemist Jose-Luis Jimenez writing for Time magazine: COVID-19 Is Transmitted Through Aerosols. We Have Enough Evidence, Now It Is Time to Act. In it, he argues that while much of the early attention has been on fomites (surface contamination) and droplets as pathways for spreading Covid-19, tranmission by aerosols may be more significant. His analogy of smoke makes it quite easy to understand:
When it comes to COVID-19, the evidence overwhelmingly supports aerosol transmission, and there are no strong arguments against it. For example, contact tracing has found that much COVID-19 transmission occurs in close proximity, but that many people who share the same home with an infected person do not get the disease. To understand why, it is useful to use cigarette or vaping smoke (which is also an aerosol) as an analog. Imagine sharing a home with a smoker: if you stood close to the smoker while talking, you would inhale a great deal of smoke. Replace the smoke with virus-containing aerosols, which behave very similarly, and the impact is similar: the closer you are to someone releasing virus-carrying aerosols, the more likely you are to breathe in larger amounts of virus. We know from detailed, rigorous studies that when individuals talk in close proximity, aerosols dominate transmission and droplets are nearly negligible.
Thinking of smoke can help guide our actions:
The visual analogy of smoke can help guide our risk assessment and risk reduction strategies. One just has to imagine that others they encounter are all smoking, and the goal is to breathe as little smoke as possible. But COVID-19 is not very contagious under most situations, unlike, for example, measles: the CDC says that 15 minutes of close proximity to a COVID-19 infected person often leads to contagion, which provides an estimate of how much “exhaled smoke” one may need to inhale for infection. Inhaling a little whiff of “smoke” here and there is OK, but a lot of “smoke” for a sustained period of time and without a mask is risky. (To be clear, actual smoke does not increase the probability of infection.)
In thinking about aerosolized Covid, Jimenez recommends avoiding crowds & indoor spaces and cutting down on proximity & duration, among other things.
We should continue doing what has already been recommended: wash hands, keep six feet apart, and so on. But that is not enough. A new, consistent and logical set of recommendations must emerge to reduce aerosol transmission. I propose the following: Avoid Crowding, Indoors, low Ventilation, Close proximity, long Duration, Unmasked, Talking/singing/Yelling (“A CIViC DUTY”). These are the important factors in mathematical models of aerosol transmission, and can also be simply understood as factors that impact how much “smoke” we would inhale.
But what about masks? N95s can block most aerosols but cloth masks have “huge” holes in them relative to aerosol particles, right? They still work in lowering risk (but not eliminating it!) because your “smoke” doesn’t travel as far when you’re wearing a mask and it filters incoming smoke (if your mask fits correctly and you’re keeping your distance).
Second, masks are essential, even when we are able to maintain social distance. We should also pay attention to fitting masks snugly, as they are not just a parapet against ballistic droplets, but also a means to prevent “smoke” from leaking in through gaps. We should not remove masks to talk, nor allow someone who is not wearing a mask to talk to us, because we exhale aerosols 10 times as much when talking compared to breathing. Everyone should be careful to not stand behind someone with a poorly fitting mask, as the curvature of an ill-fitting mask can cause aerosols to travel behind the person wearing it.
Great article, full of common sense advice backed up by science.
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