Consider:
The half-life of SARS-CoV-2 in aerosol form is said to be roughly one hour at 70°F, 20% humidity, and little sunlight.
Aerosol transmission (as opposed to macroscopic airborne droplets, droplets on surfaces, etc.) is now thought to be a significant thing.
Given these, if an infected person emits a virus-laden aerosol cloud outdoors on a dry evening with a light breeze - say, 2 on the Beaufort scale - how soon is it safe to pass through the area where said cloud was emitted?
Ha! The aerosol cloud is part of the air, not the ground. Within a couple of seconds, it will no longer be where it was emitted. In ten minutes, it'll be a mile away. After one half-life, several miles.
And, thanks to turbulence, it'll be widely dispersed. ZOMG THERE'S NO SAFE DISTANCE! REMAIN INDOORS! WE'RE ALL GONNA DIE!
Or... basically everyone who doesn't live in a space suit is constantly being exposed to traces of the virus in aerosol form, right? Which might explain the unexpected percentage of people with antibodies? Maybe a tiny exposure is enough to get detectable levels of antibodies without overwhelming the exposed individual's immune system? (How much immunity one acquires from such background-level exposure is another question entirely.)
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