COVID-19 Data Dives: Why Arguments Against SARS-CoV-2 Aerosol Transmission Don’t Hold Water

Jose-Luis Jimenez, PhD
DISCLOSURES July 30, 2020
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Jose-Luis Jimenez, PhD
I am an aerosol scientist. I have spent a lot of time examining the arguments from some that aerosols play only a very minor role in the transmission of SARS-CoV-2 — and presenting the evidence that rebuts this claim. A recent article in JAMA argues that aerosols are not an important transmission pathway for SARS-CoV-2. While the article raises good questions, the arguments against aerosols are not consistent with the best science. Here’s why I say that:
Aerosol size. 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.
Some of the arguments made are based on differences in aerosol and droplet sizes. 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. The authors write, “Droplets are classically described as larger entities (> 5 μm) that rapidly drop to the ground by force of gravity, typically within 3 to 6 feet of the source person.”
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.
This video from Ryan Davis, PhD, gives a more accurate picture of the behavior of ~50 μm aerosols in the air. Even at this size, the aerosols do not fall very rapidly to the ground. For aerosols of 5 μm to fall to the ground quickly, as shown in a short animation from the WHO, gravity on Earth would have to be 100 times larger than it is. This happens…on some stars. A 5 μm aerosol can actually stay suspended in air for 30 minutes indoors.
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. We also would not have to worry about Saharan dust reaching Florida, because much of the dust mass is composed of aerosols in this size range.
The authors further instruct us that “[a]erosols are smaller particles (≤5 μm) that…are small enough and light enough to remain suspended in the air for hours (analogous to pollen).” This is curious, given that pollen ranges in size from 15 to 200 μm. If pollen aerosols, being larger than 5 μm, actually fell to the ground within 1-2 meters, pollen allergies would not be a problem either. But pollination for many plant species would be very difficult too. Relying on medical doctors for advice about aerosols is like relying on me, an aerosol scientist, for medical advice: not a good idea.
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