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I have a character (humanoid) that has the ability to breathe underwater; either through gills or through augmented lungs but I'm not sure what starting list I should be aware of in concerning their water they'd breathe.

I know fresh and salt water is one issue but I'm sure each has their own set of problems but they're both still water (excuse pun there) and would no doubt have a lot in common nonetheless.

I know for us, the air has to be specific in composition and free from toxins but we breathe in contaminants such as pollen and other gasses. I can estimate that water would also contain contaminates of pollution and plants, but given it's much denser than air, it'll carry a lot more and suspended too which would no doubt affect the characters' ability to breathe in something 'clean'

There's also no doubt a ton of marine life; especially on the microscopic level, plants too, chemicals... but I'm not sure on how that'll impact a humanoid's ability to breath or how they can filter some of that out if they're physiology is more...augmented from a human.

So, my question is for my water-breathers, What should I keep in mind specifically before they jump into a random puddle of water and how it would affect them?

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    $\begingroup$ Remember not to dive too deep into the ocean, just because you have gills doesn't mean you won't get crushed by the extreme pressures in abyssal waters. If everything else about your body is like a buman's. you'll be at risk regarding all I'll effects of sudden pressure changes to the human body, as well as the risk of hypothermia if you don't have the necessary protection suit. Assuming long periods of permanence in the water, In fresh water you'll need to pee a lot and in salt water you'll need good kidneys to keep this water. $\endgroup$ – ProjectApex Jun 15 '20 at 21:28
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    $\begingroup$ This sounds like a list question. $\endgroup$ – John O Jun 15 '20 at 21:30
  • $\begingroup$ Don't swim in that pool - it's a sewage treatment plant. $\endgroup$ – user535733 Jun 16 '20 at 14:06
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Stirring up silt and fine particles from the bottom. Not only impacts visibility, but could be detrimental to the lungs and gills.

The amount of dissolved oxygen that is availible is also potentially a problem. Especially in still waters it could be pretty low. In fact, it is probably a lot lower than the availible oxygen in air. So the Metabolic efficiency may need to be pretty different. Apparently, from what looks to be a old dead website, here:

Water has relatively little DO (<14 mg/L) compared to the oxygen (240 mg/L) in air, so fish must be efficient in getting it. One factor that contributes to this efficiency is the unidirectional flow of water through their gills. Terrestrial animals have bidirectional flow (in and out of the same tube, the trachea) which is inefficient in that fresh inspired air mixes with stale expired air. With unidirectional flow, there is no mixing. Another factor is the countercurrent movement of blood and water. At the lamellae, blood flows one way and water the other, which insures that maximum gas exchange will occur. If water and blood moved parallel with one another there would be less gas exchange.

And build of up carbon dioxide in the water can be an issue:

Carbon dioxide is much more soluble in water than oxygen so blood can hold the amount produced with very little increase in partial pressure. That means that there is a very small pressure gradient between blood and water to push it out of the body. In order to rid themselves of carbon dioxide, fish use some physiological tricks including the use of carbonic anhydrase. Despite this low gradient between blood and water, fish in nature have little problem moving carbon dioxide out of their bodies.

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Heavy metals, for the very reason eating too much fish can kill you, breathing heavy metals through your gills is a bad Idea. And the entire planet is polluted.

Also the sea is filled with nasty parasitic insects which attach to the gills of various fish or their tongues. But I guess having an insect injecting you with pain killers so it can eat your tongue and gills can be dealt with if one pays attention to always bring a medical kit with themselves.

But it could be kind of like crossing the road, if you pay attention 99.9% of the time...it still takes just 1 single car crash to kill you.

One could touch their gills and tongue continuously, but eventually in a lifetime, it will happen at least once that they get distracted and a bug eats their tongue and gills.

We can survive in really polluted areas because our noses are filled with hairs and mucus which filters the air. If you blow your nose after a walk in Rome you will notice your mucus being black and not colorless. Thanks to our filtering system in the nose, pollution does not kill us right away, maybe it decreases our lifespan by a few years.

So your water breathing people need a similar filtering system for their gills.

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