As readers know, I have long urged, following the science, that Covid is airborne, and that its main mode of transmission is via aerosols. (I was initially persuaded by an epidemiological study, now consigned to link rot, that showed a seat diagram on a Chinese bus with the index case and how many were infected. It seemed clear that neither fomites nor hacked up droplets could be the cause, since the seats were widely separated. Many more such studies followed. No such studies followed for fomites or droplets.) On April 30 — after enormous efforts from aerosol scientists — WHO changed its guidance to reflect that Covid is airborne. On May 7, CDC followed. So, after more than a year, we have finally agreed Covid’s mode of transmission. It is to be hoped that science-based mitigation measures — especially ventilation — follow.
In this post, I will first look at what WHO and CDC actually said. Then I will look at helpful materials produced by the aerosol scientists and their allies during their battle (which may prove helpful in further battles to come, especially in schools and workplaces). Turning to institututional slash political factors, I will look at why WHO and CDC delayed so long, and how the Biden administration seems oblivious to the paradigm shift from droplets to aerosols (going to far as to push useless, even dangerous, plexiglass shields).
What WHO and CDC Actually Said
First, WHO. From “Coronavirus disease (COVID-19): How is it transmitted?“:
The virus can spread from an infected person’s mouth or nose in small liquid particles when they cough, sneeze, speak, sing or breathe. These particles range from larger respiratory droplets to smaller .
- Current evidence suggests that the virus spreads mainly between people who are in close contact with each other typically within 1 metre (short-range). A person can be infected when aerosols or droplets containing the virus are inhaled or come directly into contact with the eyes, nose, or mouth.
- The virus can also spread in poorly ventilated and/or crowded indoor settings, where people tend to spend longer periods of time. This is because  or travel farther than 1 metre (long-range).
People may also become infected by touching surfaces that have been contaminated by the virus when touching their eyes, nose or mouth without cleaning their hands.
Further research is ongoing to better understand the spread of the virus and which settings are most risky and why.
Some notes:  The word “aerosol” actually appears. Good!  The “close contact” frame does not take superspreading — as on the Chinese bus — into account.  Here, superspreading is implicit, but not stated.  Dogma. Covid rarely spreads through fomites.
Now, CDC. From “Scientific Brief: SARS-CoV-2 Transmission“:
The principal mode by which people are infected with SARS-CoV-2 (the virus that causes COVID-19) is through exposure to respiratory fluids carrying infectious virus. Exposure occurs in three principal ways: (1) of very fine respiratory droplets and  particles, (2) deposition of respiratory droplets and particles on exposed mucous membranes in the mouth, nose, or eye by direct splashes and sprays, and (3) touching mucous membranes with hands that have been soiled either directly by virus-containing respiratory fluids or indirectly by touching surfaces with virus on them.
Some notes:  Needless to say, this is a highly political document. CDC seems to need to think of aerosols as “respiratory fluids,” bless their hearts.  The word “aerosol” actually appears. Good!  Droplet dogma.  Fomite dogma. Note that CDC carefully does not say which mode is the primary mode.
And CDC again, for the layperson, in “How COVID-19 Spreads“:
COVID-19 spreads when an infected person breathes out droplets and very small particles that contain the virus. These droplets and particles can be breathed in by other people or land on their eyes, noses, or mouth. In some circumstances, they may contaminate surfaces they touch. People who are closer than 6 feet from the infected person are most likely to get infected.
Some notes:  The word “aerosol” is suppressed.  Fomite dogma.
Since this is CDC, we expect the deliverable to be sloppy and half-assed, and so it proves. From the University of Maryland School of Public Health, “Statement of Scientists in Response to CDC’s May 7th Announcement Recognizing Inhalation Exposure“:
[The “How COVID-19 Spreads” document] will lead people to continue to think that maintaining distance is sufficient to prevent transmission.
We know that transmission at distances beyond 6 feet occurs because of superspreader events, careful studies of smaller outbreaks, and the physics of aerosols. It can easily happen indoors in a poorly ventilated environment, when people are not wearing masks..
There is clear consensus among aerosol scientists and epidemiologists that inhalation of small aerosol particles, including at distances of greater than six feet, is a major driver of the COVID-19 pandemic. To slow transmission and save lives, it is crucial that CDC update its guidance and recommendations to address and highlight the importance of improved ventilation and using NIOSH approved respirators, especially in indoor locations where the virus may accumulate in the air. .
Moreover, OSHA must issue an Emergency Temporary Standard to control workplace exposures, including inhalation exposure, as soon as possible in order to protect workers and reopen the economy safely.
Nevertheless, CDC did speak the unspeakable word — aerosols! — so that is a victory.
Helpful Materials for Air Breathers
I cannot summarize the history and politics of the battle to recognize aerosol transmission because I was not in those rooms. However, I can highlight the literature that I found most illuminating along the way. (As Mr. Rogers says: “Look for the helpers.”). The literature falls into two buckets: Scientific Communications, and Scientific Papers.
First, University of Colorado aerosol scientist masterfully circumvented the United States press with this article in the English language version of El Pais: “A room, a bar and a classroom: how the coronavirus is spread through the air.” It’s one of those mobile-friendly interactive pages, but it is still worth a read even today. Here is a classroom situation:
I saw this paper cited many times in discussions of school reopenings on the Twitter, so at least some parents may have been able to protect their children from CDC’s miserably inadequate school reopening guidance, which did not consider aerosol transmission in the United States, not even in its scientific references.
Second, this wonderful article in the Lancet, “Ten scientific reasons in support of airborne transmission of SARS-CoV-2.” Here are two of the 10 points. Recall that neither droplets nor fomites can give an account of superspreading events:
First, superspreading events account for substantial SARS-CoV-2 transmission; indeed, such events may be the pandemic’s primary drivers. Detailed analyses of human behaviours and interactions, room sizes, ventilation, and other variables in choir concerts, cruise ships, slaughterhouses, care homes, and correctional facilities, among other settings, have shown patterns—eg, long-range transmission and overdispersion of the basic reproduction number (R0), discussed below—consistent with airborne spread of SARS-CoV-2 that cannot be adequately explained by droplets or fomites. The high incidence of such events strongly suggests the dominance of aerosol transmission.
Sixth, viable SARS-CoV-2 has been detected in the air. In laboratory experiments, SARS-CoV-2 stayed infectious in the air for up to 3 h with a half-life of 1·1 h.12 Viable SARS-CoV-2 was identified in air samples from rooms occupied by COVID-19 patients in the absence of aerosol-generating health-care procedures13 and in air samples from an infected person’s car. Although other studies have failed to capture viable SARS-CoV-2 in air samples, this is to be expected. Sampling of airborne virus is technically challenging for several reasons, including limited effectiveness of some sampling methods for collecting fine particles, viral dehydration during collection, viral damage due to impact forces (leading to loss of viability), reaerosolisation of virus during collection, and viral retention in the sampling equipment. Measles and tuberculosis, two primarily airborne diseases, have never been cultivated from room air.
Third, this handy chart from the British Medical Journal, “Two metres or one: what is the evidence for physical distancing in covid-19?”:
(Tricia Greenhalgh has aggregated translations of this chart here.) I like this chart first, because it’s small (as opposed to that ludicrously ginormous CDC chart I printed the other day) and second because its simple: Actions (singing, breathing, masking, not masking) on the vertical axis, built environment on the horizontal. The famous Skagit County choir practice superspreading event, for example, is at bottom right (red, unsafe). Singing, no masks, prolonged contact, high occupancy, poorly ventilated. A silent walk with your friend in the park, both masked, would be top left (green, safe)
We have all linked to many, many papers on aerosols, but this paper (I hope) is extraordinary: “Indicators for Risk of Airborne Transmission in Shared Indoor Environments and their application to COVID-19 Outbreaks.” (I say “I hope” because the math is over my head; I hope some maven in the readership will give it a go.) In essence, the authors have devised a formula for calculating the risk of indoor transmission, based on a set of parameters. They then aggregated and tabulated a large number of aerosol studies and threw them against the formula, with parameter values taken from real events as documented by epidemiological studies. From the body of the paper:
An important advantage of the simplified risk parameters is that their values can be calculated
for outbreaks that are documented in the scientific literature. Values for documented COVID-19 outbreaks are shown in Table 1 (rE and rB are estimated based on the likely types of activities in each case (33, 37, 44), see Table SI-2 for typical values). Also included are values for outbreaks documented in the literature for tuberculosis and measles, which are widely accepted to transmit through the air, and an influenza outbreak that was clearly due to airborne transmission.
The authors then took the formula and turned it into a calculator:
12/ If you want to calculate the risk parameter for your situation, you can do so with the COVID-19 Aerosol Transmission Estimator, freely available online.
— Jose-Luis Jimenez (@jljcolorado) May 2, 2021
Here is the calculator. It made me scream and run; the calculator needs to be made usable by, say, an HVAC person in a school (and not a high-priced consultant). But that’s what software is for, so this is a mere implementation detail.
The Disgraceful Response of the CDC (and WHO)
Here I can do no better than to quote a great slab of Zeynep Tufecki’s “Why Did It Take So Long to Accept the Facts About Covid?” in the New York Times, which you should read in full:
Why did it take so long to understand all this?
One reason is that our institutions weren’t necessarily set up to deal with what we faced. For example, the W.H.O.’s Infection Prevention and Control (I.P.C.) global unit primarily concentrates on health care facilities. Many of the experts they enlisted to form the Covid-19 I.P.C. Guidance Development Group were hospital-focused, and some of them specialized in antibiotic-resistant bacterial infections that can spread wildly in health care facilities when medical personnel fail to regularly wash their hands…. Further, in some countries like the United States, they tend to have extensive engineering controls to dampen infections, involving aggressive air-exchange standards, almost like being outdoors. This is the opposite of modern office and even residential buildings, which tend to be more sealed for energy efficiency. In such a medical environment, hand hygiene is a more important consideration, since ventilation is taken care of. Another dynamic we’ve seen is something that is not unheard-of in the history of science: setting a higher standard of proof for theories that challenge conventional wisdom than for those that support it.
As part of its assessment of the virus’s spread, the W.H.O. asked a group of scientists last fall to review the evidence on transmission of the coronavirus. When reviewing airborne transmission, the group focused mostly on studies of air samples, especially if live virus was captured from the air, which, as mentioned above, is extremely hard. By that criterion, airborne transmission of the measles virus, which is undisputed, would not be accepted because no one has cultivated that pathogen from room air. That’s also true of tuberculosis. And while scientists, despite the difficulties, had managed to capture viable SARS-CoV-2 in three studies that I’m aware of, the review noted that the virus was detected only intermittently in general, disputed whether the captured live virus was infective enough and ultimately said it could not reach ‘firm conclusions over airborne transmission.’ The lead author and another senior member of the research group previously said they believed transmission was driven by droplets.
The skepticism about airborne transmission is at odds with the acceptance of droplet transmission. Dr. Marr and Joseph Allen, the director of the Healthy Buildings program and an associate professor at Harvard’s T.H. Chan School of Public Health, told me that droplet transmission has never been directly demonstrated. Since Dr. Chapin, close-distance transmission has been seen as proof of droplets unless disproved through much effort, as was finally done for tuberculosis.
Another key problem is that, understandably, we find it harder to walk things back. It is easier to keep adding exceptions and justifications to a belief than to admit that a challenger has a better explanation.
…[T]he initial public health report on the [Skagit County] Mount Vernon choir case said that it may have been caused by people “sitting close to one another, sharing snacks and stacking chairs at the end of the practice,” even though almost 90 percent of the people there developed symptoms of Covid-19. Shelly Miller, an aerosol expert at the University of Colorado Boulder, was so struck by the incident that she initiated a study with a team of scientists, documenting that the space was less full than usual, allowing for increased distance, that nobody reported touching anyone else, that hand sanitizer was used and that only three people who had arrived early arranged the chairs. There was no spatial pattern to the transmission, implicating airflows, and there was nobody within nine feet in front of the first known case, who had mild symptoms.
The Biden Administration Stumbles
Finally, we come to the Biden administration, which seems completely unaware that a paradigm shift in our understanding of Covid’s mode of transmission is occurring. I presented this video once, but it still frosts me, so I’m going to present it again:
— The Hill (@thehill) May 3, 2021
First, as should now be obvious, plexiglass screens don’t protect school children from aerosols, because the aerosols, just as cigarette smoke would, go around the barriers.
Second, plexiglass screens may actually increase the danger of Covid transmission. Here is a handy chart from Science, “Household COVID-19 risk and in-person schooling“:
Here is a video from one of those helpful aerosol scientists explaining why:
I have been lots of questions about plexiglass barriers–this is the best visualization that I have seen. They are designed to block big droplets produced in sneezes and coughs. Aerosols float right around them and can actually concentrate near a person sitting on the other side. https://t.co/0jKd0fk32s
— Kimberly Prather, Ph.D. (@kprather88) December 17, 2020
And we have a real-life example. From medRvix: “Weekly SARS-CoV-2 screening of asymptomatic students and staff to guide and evaluate strategies for safer in-person learning,” the Conclusion:
In this school district with layered mitigation measures, in-school transmission was rare. The program identified a cluster with in-school staff-to-staff transmission and spurred enhancement of safety strategies.
And here are some of those enhancements:
First, review of heating, ventilation, and air conditioner (HVAC) systems, including assessment of airflow in the main office using smoke, suggested .
As I read the study, then, plexiglass dividers actually contributed to an outbreak.
When I see that [family blogging] Biden video, I want to ask “Who did this?” The room is poorly ventilated. Plexiglass dividers are at best useless, at worst dangerous. It’s a dangerous place for students to be! And yet we have a President “modeling behavior,” as we say, based on an obsolete paradigm for transmission. So all over the country school districts are going to be buying plexiglass barriers instead of fixing their ventilation systems! Ron Klain, Biden’s chief of staff, was sold to us as a pandemic expert based on his Ebola Czardom. Was Klain asleep? Not paying attention? Advised by some droplet goon at Walensky’s CDC? Fondling his West Wing box set? Who knows. What a debacle!
Piling debacle upon debacle, we know have this idiotic statement by Fauci. From CNN:
Sunday on ABC News, Fauci was asked whether it’s time to start relaxing indoor masks requirements. Fauci replied, “I think so, and I think you’re going to probably be seeing that as we go along, and as more people get vaccinated.”
Madness, because as aerosol theory predicts, and fomite and droplet theories do not, indoors spaces are the most dangerous spaces (see Greenhalgh’s helpful chart above). We don’t know, for any given space, how many people within in will be vaccinated. We also do not know how many have been vaccinated and are still capable of asymptomatic spread — and hence, in a closed space, of becoming superspreaders. Fauci should be focusing on ventilation, which actually makes people safer, instead of playing Philosopher King and using some people’s desire to unmask to manipulate them into getting vaccinated.
Concluding, the intellectual victory needs to be fought out on the ground of mitigation (which, above, both Biden and Fauci butchered). From the Toronto Globe and Mail:
A transition to an airborne approach revolves around a simple and powerful concept: COVID spreads by breathing shared air, and infectious air accumulates indoors.
Primary mitigation strategies shift from two metres distancing and handwashing to ventilation and high-quality masks. Monitor ventilation using portable carbon dioxide monitors, open windows and emphasize improvements in ventilation or air filtration when ventilation systems are poor. Distancing remains important but high-quality respirators should always be worn while indoors. All breaks and meals where masks will be removed must be moved outdoors or, when outdoors is not possible, in large well-ventilated rooms with maximal distancing and small numbers of people.
While airborne transmission is now acknowledged, changes to infection prevention strategies and efforts to increase public awareness are lacking. These changes will reduce onward transmission in high-risk settings and provide opportunities for a return to normalcy in some spheres: outdoor activities (where aerosols are rapidly dispersed) and even some indoor spaces with guidance on how to eliminate aerosol combined with proper masking.
Shifting to an airborne prevention approach will be life-saving for people working in high-risk essential jobs, and for the families to whom they bring home the infection. It will also impact overall epidemic control and help guide a safer more sustainable economic reopening.
One of the nice things, of course, about mitigation strategies like this is that — unlike today’s vaccines — they’ll work when the next airborne pandemic comes along. We sure dodged a bullet that Covid wasn’t as contagious as measles. Eh?
 I can say that the aerosol advocates reminded me very much of MMT advocates. It may be that the pleasant group dynamics gave both thought collectives some leverage to effect their desired paradigmatic shifts.