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I have a setting where I want the atmosphere to basically be filled, heavily and consistently, with a bunch of small, airborne bacteria-like cells/microscopic biomatter. For the sake of keeping the question focused on the effects of the actual environment at hand, let's disregard any and all effect these cells themselves may actually have on the inhabitants of my setting who are inhaling it so much, and just assume that they have no functional effect for now; they simply pass into the lungs and bloodstream, and then quickly decay or get disposed of.

What amount of this airborne bacteria/cells can I put into the air in my setting that the inhabitants breathe, without it being so heavily present that it literally "clogs" the breathable material in the atmosphere (or perhaps more importantly, without "clogging" the respiratory and circulatory systems that then have to deal with all of it). For example if I want inhabitants in my setting to inhale up to hundreds of thousands or even millions of these individual cells in a single breath, is that possible to design the environment to do, or is that simply too many of a bacterium-sized object to fit safely into the scale of a single breath?

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    $\begingroup$ "they simply pass into the lungs and bloodstream", That's not how lungs operate. Some particle sizes get lodged in the lungs ie silicosis, black lung disease. Other material gets stuck in the mucus membranes and gets moved to where it get be broken down ie down the esophagus, out the nose etc.. A large portion is just get exhaled back out. There is systems in place to prevent bacteria from entering the blood stream because it is almost always bad news if that happens. $\endgroup$ Aug 24, 2023 at 18:07
  • $\begingroup$ If you're ignoring any and all impact of the cells themselves, then look at the way lungs and atmosphere interact directly. Our Earth’s atmosphere comprises roughly 78% nitrogen, 21% oxygen and some argon and other gases, including carbon dioxide and methane, water vapour and neon. If your small, airborne bacteria-like cells change much of that, and particularly the oxygen or carbon di-oxide, people like us will die. Below 19.5% oxygen, science says air is 'oxygen deficient.' Can you do the maths? $\endgroup$ Aug 26, 2023 at 22:08

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A hundred thousand to a million per breath.

There are limits on airborn bacteria in terms of size. Bacteria and such can survive in the air by forming what is called bioaerosol particles and there's generally just around 100k per meter cubed, while a normal breath will be just half a liter, so a couple hundred is normal, while 6000 would be a deep breath.

It's hard to work out the maximum livable density of bacteria, but the highest concentration of smog is just 100 times the norm. This would be when the air is thick with visible bacteria particles and you can't see more than a few meters away clearly.

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    $\begingroup$ A hundred thousand to a million per what? $\endgroup$ Aug 25, 2023 at 1:33
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    $\begingroup$ OP asked how many airborne bacteria per breath. I said, a hundred thousand to a million. $\endgroup$
    – Nepene Nep
    Aug 25, 2023 at 10:37
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    $\begingroup$ They didn't ask how many bacteria per breath. They asked "How many airborne bacteria can I put into the air while keeping it breathable?" The closest they got to asking how many bacteria per breath was was "For example if I want inhabitants in my setting to inhale up to hundreds of thousands or even millions of these individual cells in a single breath, is that possible to design the environment to do, or is that simply too many of a bacterium-sized object to fit safely into the scale of a single breath?", but "a hundred thousand to a million" isn't a sensible answer to that question either. $\endgroup$ Aug 25, 2023 at 10:43
  • $\begingroup$ I added per breath, since their question was in the post. $\endgroup$
    – Nepene Nep
    Aug 25, 2023 at 11:48
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Your upper limit is not breathable limit but floral limit. With enough airbone bacteria to clog lungs You do not have enough light on surface to have foliage. Second thing is that bacteria will be water droplets agregtion point on middle and high altitudes making lots of clouds. And rain will clean up low altitudes from bacteria. That can lead to make layer of hot air in middle altitude where will be lots of bacteria and sparse bacteria next to dark, cold ground.

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  • $\begingroup$ "it literally 'clogs' the breathable material in ---the atmosphere---" your lungs, and you drown. But not before you and everything else dies due to lack of sunlight, +1. $\endgroup$
    – Mazura
    Aug 24, 2023 at 17:21
  • $\begingroup$ Maybe the bacteria photosynthesize. Perhaps as the weather mixes the air, the bacteria on the top get cycled around to the bottom, where things breathe them in and digest them, I guess. $\endgroup$
    – Erhannis
    Aug 25, 2023 at 6:22
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Depends on the respiratory system - there are literally organisms on earth that breath water. Why not breath bacteria?

If your organisms evolved a respiratory system that basically has a big filter and the air passes tangentially to that filter (self-cleaning), it could work.

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There is no real upper limit.

They can form a fog if you want them to. If people evolved in these conditions they can deal with it.

This article https://en.wikipedia.org/wiki/Aeroplankton says:

Small, drifting aeroplankton are found everywhere in the atmosphere, reaching concentration up to 10^6 microbial cells per cubic metre. ... Aeroplankton is made up mostly of microorganisms, including viruses, about 1,000 different species of bacteria, around 40,000 varieties of fungi, and hundreds of species of protists, algae, mosses, and liverworts...

Mostly we have no idea. Impact seems to be unclear:

While the chemical components of particulate matter pollution and their impacts on human health have been widely studied, the potential impact of pollutant-associated microbes remains unclear. Airborne microbial exposure, including exposure to dust-associated organisms, has been established to both protect against and exacerbate certain diseases.

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I think the limit would be 2 things.

When sun stops reaching earth.

and

When you cant see your hands

Ill let you decide while giving some insight into it, theres 1070 particles/cm3 in a dense fog with visibility under 30 meters. Meaning in your cenario in a dm3 there are 1 070 000 millions bacteria meaning a full breath would take 500 000 ~ bacterie to the lungs, if you want a million per breath it would need to be 2 000 000 bacteria per Dm3 which would be 1 000 000 ~ per breath.

Ofc with both these numbers plants wont grow at all due to lack of sun, and the world would be cold. With 2 000 000 bacteria per dm3 it would be impossible to see anything over 10m.

The air would feel heavy, like high humidity but stronger, but it souldnt cause diference in breathing once we adapt a little to it.As for "Clogging" the lungs, you would need to breath in something that was bigger 10 ug in infamable quantities over some time, like a forest fire conditions.

Possibly the visibility would have been way better since im assuming 40-50ug in size for fog and for them to enter the bloodstream/lungs they would need to be 10ug max.

So i would say that the limit would be arround 10 000 000 (low visibility somewhat annoying breathing) and 50 000 000 (that would certainly have bad bad side effects i cant really understand what might come from it).

This isnt an opinion sources for everything : https://www.mdpi.com/2073-4433/11/3/258

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