Monday, April 5, 2021

Airborne Transmission - Once Again!


I thought I would take time for a rare celebration on this blog.  Most of my writing is about probabilities and uncertain outcomes. In many cases I am responding to the same tired arguments from people who don’t understand science, biology, medicine or psychiatry. Those positions generally result in some political attacks based on that lack of understanding or some specific political agenda. The position I am referring today is the airborne transmission of viruses. Although it seems like a straightforward scientific issue it has led to as much controversy as any psychiatric topic. Despite a significant amount of literature out there on airborne spread, there has been nothing but resistance to the concept.

Nowhere was the resistance more evident than the advent of the current SARS-CoV-2 pandemic. Initially the message was that the virus was spread by fomites or intermediate size droplets that fall within a few feet following a cough or a sneeze.  Accordingly, social distancing at more than a few feet, decontaminating hands and surfaces were recommended to counter this mechanism of transmission. Many experts claim that most respiratory viruses with very few exceptions are transmitted this way. Those same experts claim that airborne transmission of viruses in smaller droplets travelling much longer distances was controversial at best. All of those conflicting ideas led to recommendations for no masks in February of 2020 followed by recommendations for masks in the next two months.  The mask recommendations occurred in the context of widespread shortages of personal protective equipment (PPE) for health care workers.   

I posted my qualifications on the matter (2 Avian Influenza Task Forces earlier in this century, being subjected to multiple respiratory virus epidemics at work despite rigorous hand washing, and studying the available engineering and viral data, and lengthy discussions with HVAC experts) and began to write about it on this blog.  My perspective is clearly that respiratory viruses are airborne and therefore will not be stopped by handwashing alone, that there are clearly engineering approaches to stop respiratory viruses that will work much better than just handwashing, and that there should be a major research and development effort on environmental designs to minimize and even stop respiratory viruses in homes and public building. In fact, as I type this I have selected a UVC device to be installed in my home HVAC system and it will probably be installed in the next month or two.  Many of those posts on this blog can be found here or by using the search term “airborne” in the search box.

The victory lap today occurs with a press release from the CDC today that I consider a bombshell in terms of the airborne transmission concept.  The press release is a quick read but it highlights why surface contamination is unlikely to be a significant factor:

“Quantitative microbial risk assessment (QMRA) studies have been conducted to understand and characterize the relative risk of SARS-CoV-2 fomite transmission and evaluate the need for and effectiveness of prevention measures to reduce risk. Findings of these studies suggest that the risk of SARS-CoV-2 infection via the fomite transmission route is low, and generally less than 1 in 10,000, which means that each contact with a contaminated surface has less than a 1 in 10,000 chance of causing an infection.”

And further:

“The principal mode by which people are infected with SARS-CoV-2 is through exposure to respiratory droplets carrying infectious virus.”

This information has been slowly presented over the course of the past several months.  For example, Dr. Fauci mentioned on several news outlets that cleaning all of the mail and groceries was not necessary because it was not considered a main route of transmission. A logical inference from that statement is why there is a concern about any surfaces at all unless there is a person with a known infection close by.  And by extension, if surface contamination is not that much of a problem why the concern about accidentally touching your face?  As Dr. Fauci typically states we now have the science behind the transmission and the recommendations can be adapted to the new findings.

The CDC press release does not come right out and say airborne transmission.  They continue to say respiratory droplets are the predominate mode of spread and the old document on respiratory droplets says nothing about differentiating between moderate sized droplets that typically fall to the ground within a 6-foot radius of where they are generated or airborne droplets that are lighter, spread past 6 feet from the generation site and remain suspended for longer periods of time.

Some of the comments on the press release have been much more definitive. The only reference to this post has a good timeline on the airborne controversy and this quote from atmospheric chemist Jose-Luis Jimenez: “If we took half the effort that’s being given to disinfection, and we put it on ventilation, that will be huge.”  In the same reference Germany has invested a half billion dollars in improving ventilation and indoor air quality.

Overall, it appears that the CDC is slowly coming around to the idea that respiratory viruses are transmitted via airborne routes, but some resistance is still evident in the press release they link to an earlier non-descript respiratory droplet transmission document.  There are many potential advantages to fully backing the airborne transmission concept (in addition to the available science).  Research and development is at the top of the list. In an early blogpost, I pointed out that UV decontamination was routine in buildings when I was a kid in a small town in northern Wisconsin.  The currently available UVC is much safer and very effective for killing airborne biological particles. From a clinical trials perspective, deployment of these systems on a large scale and following the number of respiratory infections in facilities with and without the technology seems like a fairly basic experiment.

It is also interesting to consider the resistance. There is undoubtedly politics in science and that can be a factor. There may be a medical intervention bias. In other words, we need some magical intervention like a vaccine, antiviral medication, or general polypharmaceutical modality that can either cure or prevent the excessive morbidity and mortality from respiratory viruses.  The track record there is some wins and many losses.  Every year various populations around the world are subjected to significant effects from flu-like illness that are nowhere as lethal as SARS-CoV-2.  Remarkably – everyone accepts this state of affairs until a more lethal virus comes around and affects a larger group of people.  There is politics as usual leading to irrational attitudes about viruses and physical interventions.  The appropriate environmental interventions may make mask refusers irrelevant at some point in the future.

The bottom line of today’s release is good news for all of the airborne virus crowd and I definitely consider myself in that crowd. I would still like to see the CDC modify their position on transmission in respiratory droplets and I think that is coming.  But most of all, I would like to see us get serious about using environmental measures to limit the exposure and spread of all respiratory viruses including the current one that has killed far more Americans than any influenza epidemic since 1918.


George Dawson, MD, DFAPA



1:  Lewis D. Why indoor spaces are still prime COVID hotspots. Nature. 2021 Apr;592(7852):22-25. doi: 10.1038/d41586-021-00810-9. PMID: 33785914.

2: Dietrich WL, Bennett JS, Jones BW, Hosni MH. Laboratory Modeling of SARS-CoV-2 Exposure Reduction Through Physically Distanced Seating in Aircraft Cabins Using Bacteriophage Aerosol — November 2020. MMWR Morb Mortal Wkly Rep. ePub: 14 April 2021. DOI:

3: Greenhalgh T, Jimenez JL, Prather KA, Tufekci Z, Fisman D, Schooley R. Ten scientific reasons in support of airborne transmission of SARS-CoV-2.  The Lancet (online).  Published 4/15/2021.  Current link

4: Tang JW, Bahnfleth WP, Bluyssen PM, Buonanno G, Jimenez JL, Kurnitski J, Li Y, Miller S, Sekhar C, Morawska L, Marr LC, Melikov AK, Nazaroff WW, Nielsen PV, Tellier R, Wargocki P, Dancer SJ. Dismantling myths on the airborne transmission of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2). J Hosp Infect. 2021 Apr;110:89-96. doi: 10.1016/j.jhin.2020.12.022.  Current Link

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