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Comment by AnimalMuppet | Hacker News Reader

Wait a minute.

> But the researchers instead found evidence for the opposite: “The key factor for infection was the direction of the airflow,” with downstream individuals being most at risk—a result consistent with the thesis that COVID-19 is transmitted primarily through the ballistic transmission of large respiratory droplets.

If the direction of airflow is the key factor, then the droplets have to be moving with the airflow, and therefore not ballistic.

Am I missing something?

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I believe the reasoning is that aerosol would have ultimately spread everywhere, including on the AC itself, while ballistics can be heavily influenced by airflow, but are still limited in range.

I think the definition of ballistic means that gravity is the dominant force governing its path through space. So if it is "heavily influenced" by airflow it is by definition not ballistic.

Then again, it's not aerosolized as it didn't diffuse homogenously into the space. It's somewhere in between.

Eh, different fields can have different definitions for the same word, and that's okay. If I tell a chemist that carbon is a metal, I would get a funny look. If I tell an astrophysicist that carbon is a metal, it is perfectly reasonable. Within the field of astronomy, metals are any element other than hydrogen and helium. There may be a similar difference in definitions here

For those who are curious, you can read more here: https://en.wikipedia.org/wiki/Metallicity

You've really got three regimes. Really big things are ballistic in your sense that the dominant force is gravity. Smaller things are also sensitive to bulk air movement. Really small things aren't affected much by gravity, and brownian motion is more important.

When these guys are saying "ballistic", they mean that the droplet is big enough that gravity is important, and that it's going to hit the ground in a reasonable amount of time and not hang around in the air until convection/diffusion spread it evenly throughout the room.

Yeah, this explanation was messed up. Also, if you look at the graphic associated with this, it seems like people both upstream and downstream of the individual were infected and it is not clear if some of them had the infection simultaneously or were actually infected by the first person. Lots of questions here.

speaking of ballistics. i wonder, is there an easy way to increase the surface tension on these droplets? Maybe an inhaler or something that makes these droplets large and fall to the ground faster?

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