A recent article in JAMA argues that aerosols are not an important transmission pathway for SARS-CoV-2.
Most important, a good understanding of aerosol physics, airflow, and dilution is needed to interpret the behavior of potentially infectious aerosols in complex real-world situations.
Both are particles of solid or liquid material in air, with the difference being that aerosols stay suspended for longer times (minutes to hours indoors), while droplets behave ballistically and fall to the ground quickly (in seconds).
To be sure, size is the most important property of particles, and because mass increases with the cube of the diameter, the fate and transmission mode of aerosols and droplets change dramatically with size.
However, the actual size of droplets that fall to the ground that quickly correspond to sizes larger than 50 μm, so 10 times the size and 1000 (!) times the mass given in the article. This fundamental error has been repeated for decades in guidance from the Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO) and in medical papers, despite the correct physics having been figured out by Wells in 1934 and the error having been pointed out many times by other scientists.
Even at this size, the aerosols do not fall very rapidly to the ground.
If 5-μm aerosols fell to the ground as stated by the authors and as shown in the WHO's video, we would not have to worry about aerosol (particulate matter) pollution, because a large fraction of it would fall to the ground so quickly.
Relying on medical doctors for advice about aerosols is like relying on me, an aerosol scientist, for medical advice: not a good idea.
When talking close to another person, we inhale a fraction of the respiratory aerosols that they exhale (that may contain the virus).
But quantitative studies with very detailed physics have been published that estimate exposure to droplets and aerosols in a close proximity situation (often referred to as "close contact," although physical contact is typically not involved).
Droplets have larger volume to begin with, but the aerosols are more finally divided (providing more chances for inhalation) and stay much longer in the air, so that the physics of aerosol and droplet motion overwhelmingly favor aerosol exposure.
Therefore, aerosols probably dominate "close contact" transmission when talking.
For many decades it was thought that tuberculosis was transmitted through droplets and fomites, because transmission seemed to be strongly favored extended "close contact." It was later proven that tuberculosis can only be transmitted through aerosols, highlighting the difficulty of concluding which route may dominate on the basis of an observation of favorable transmission under "close contact.".
Some people argue that the clear effectiveness of limiting "close contact" through social distancing to reduce SARS-CoV-2 transmission demonstrates that droplets, which fall to the ground close to the person, are the dominant mode of transmission, and that the same observation disproves aerosols as an important source of infection.
That "close contact" is a major mode of transmission of SARS-CoV-2 does not disprove aerosols.
They conclude, "This pattern seems more consistent with secretions that fall rapidly to the ground within a narrow radius of the infected person rather than with virus-laden aerosols that...remain suspended in the air at face level for hours where they can be inhaled by anyone in the vicinity."!
This strongly suggests that it is the known higher respiratory aerosol generation during singing that is driving the increased transmission.
Although talking results in lower respiratory aerosol generation, it can make up for it with longer time in the same room, low ventilation etc.
Why is there such a disconnect in the understanding of the role of aerosols in SARS-CoV-2 transmission.
And information about the role of "droplets" being dominant is concluded from whether transmission that can be reasonably well managed with "droplet" PPE and procedures.
The physics of aerosols and droplets are much more complex and nuanced, and the real behavior of aerosols and droplets does not align with what the different types of PPE protect against.
Although the virus is small (about 0.12 μm), it is encased in larger aerosols containing respiratory fluid or saliva.
Although data are scarce, we suspect that aerosols in the range of 1-10 µm produced in speech (not coughing or sneezing) dominate aerosol transmission of SARS-CoV-2.
This is true for pollution aerosols, with a typical size of 0.3 µm.
But aerosols in the supermicron range are reasonably well filtered by well-worn surgical masks, potentially explaining why "droplet PPE" is quite effective against SARS-CoV-2 transmission in hospitals (as the authors argue may be the case), even if aerosol transmission is one of the main routes.
A final argument that is often made by scientists skeptic of the importance of aerosol transmission, including those advising the WHO (although not in this particular paper), is that viable SARS-CoV-2 has not been isolated from the air.
It makes no sense to keep demanding definitive proof of viable virus in room air before accepting aerosol transmission of SARS-CoV-2, in the face of mounting evidence supporting this route, the lack of strong arguments against it, and of actually less evidence for the other routes.
But those are still the accepted routes of transmission, whereas aerosols are still described as much less important.
As I said at the top, I have spent a lot of time searching for the arguments to counter those who argue that aerosols play at most a very minor role in the transmission of SARS-CoV-2.
That only a major role for aerosols explains the observed patterns of transmission is a final, powerful argument.
It is unfortunate that key deciding bodies in major health organizations, that have concluded that SARS-CoV-2 does not transmit through the aerosol route, lack researchers with expertise in aerobiology, aerosols, and building science, among other disciplines.
More important, it is critical that medical researchers collaborate with aerosol researchers and related fields.
Although we know enough to strongly suspect that SARS-CoV-2 has a substantial component from aerosol transmission, there is a lot that we do not know and that we need to learn to fight this pandemic, future ones, and other respiratory diseases that also probably have an aerosol component in their transmission.
The research of the Jimenez group centers on atmospheric aerosols and aerosol instrumentation
Cite this: COVID-19 Data Dives: Why Arguments Against SARS-CoV-2 Aerosol Transmission Don't Hold Water - Medscape - Jul 30, 2020
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