Because we’re also constantly filling rooms with the CO2 that we exhale.

The CO2 meter acts as a proxy for how much exhaled “stuff” is lingering about.

So if your CO2 meter shows that the room is at ~400ppm [:’(], rather than 800 or 1000, then you can deduce that the room is well ventilated.

I’m other words, whatever ventilation removed the excess CO2 most likely also removed the aerosols at the same time.

Considering atmospheric CO2 levels are around 400ppm you're not going to keep it that low indoors unless it's full of open windows.

Yup. I have all of my windows open right now, it’s summer. Co2 tends to be around 500. Even outdoor city air in my location is more like 450, though of course my meter could be biased high.

That’s what I was getting at. 400 is the lower bound, if you have many windows open on a breezy day in the country.

The tear emoticon was a nod to how high that baseline is.