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> I think we use them as a proxy for o2 levels (same as our bodies do).

Probably. ISTR that depriving a body of oxygen results in a different response than overloading the body on CO2. It's why if you completely displace all air in the room with CO2, people choke, panic, etc, but if you use Nitrogen, people just keel over dead without realising it.

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The evolved response to CO2 is part of the human body’s ability to filter and remove CO2 via the respiratory system. AFAIK we don’t have similar capacity for Nitrogen because it’s not a primary waste product of that system.
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Dissolving CO2 in water creates some carbonic acid (H2CO3) that will decompose back to water and CO2 when the CO2 concentration drops. Blood has a fair bit of water, and carbonic acid is much easier to detect than oxygen or nitrogen gas

We evolved to detect CO2 because that's by far the easiest thing to detect that's still a reasonable proxy for the performance of our respiratory system

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I'm pretty sure that in a room where we replaced nitrogen with co2, we would be dead even if O2 concentrations were the same. Something about partial pressure. I notice AI explanations agree with me (not going to copy and paste them).
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EDIT: ignore this; I was confused / misinformed

It's about pH. CO2 creates carbonic acid when it dissolves in water. Your blood pH, in turn, controls how much you feel like you need to breathe. So with high CO2, your respiration rate slows down, and that can lead to low oxygen levels.

Note that the physiology and biochemistry of this is complicated (e.g. blood is a very good pH buffer and it's actively regulated by kidneys etc) and it's very much a nascent field of research, so I think AI will be overconfident and hallucination-prone.

Source: I worked in high-co2 caves for my PhD so have read about this a lot. I always carried a CO2 monitor. Our rule was to get out if we saw 20,000 ppm or greater. I spent thousands of hours above 10,000ppm.

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My medical student flatmates were talking a lot about acidosis and alcalosis :)

It was the first time that I heard about them. These basically never happen if your body and environment are halfway decent, but they are important in exceptional situations.

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Wouldn't high CO2 make you breath faster?
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Interesting, the linked article does say that.

Pretty sure I learned the effect was the opposite (high CO2 --> slower respiration). Note that that was ~15 years ago when I would have read that. Maybe I just misunderstood, or thinking has changed.

edit: reading now I see I was wrong about this. Thanks for the correction!

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You are right about the pH implications, but respiratory acidosis leads to hyperventilation, not hypoventilation. CO2 will kill you regardless of oxygen supply.

https://en.wikipedia.org/wiki/Hypercapnia

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This is extremely wrong: CO2 impairment kicks in around 1000 ppm[1] possibly lower.

You can hit this breathing by yourself in an unventilated 3x3m room (literally measured in my house).

1 https://pmc.ncbi.nlm.nih.gov/articles/PMC4892924/

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You hit it even easier when driving in a car with the internal circulation turned on to keep nasty fumes out.
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In a car with Recirculation Mode on Levels routinely spike between 1,500 and 4,000 ppm within 20 to 30 minutes.

I wonder how many driving accidents can be saved by having a co2 monitor in the car.

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What makes you think that?
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We do.
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Huh. You don't know that, and are making it up. It's almost certainly false.
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