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this post was submitted on 22 Jun 2023
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Not actually correct in this case. Hypoxia is only painless if oxygen is displaced with an inert gas like nitrogen. Our bodies detect low oxygen indirectly via chemoreceptors that detect the increase in blood acidity (respiratory acidosis) induced by high carbon dioxide (hypercapnia).
As humans breathe in a sealed environment, oxygen is replaced with CO~2~. Hypercapnia is what causes the panic and pain of drowning prior to inhalation of water. Consciousness is lost mere seconds after water inhalation.
Drowning and hypercapnic asphyxiation are essentially the same experience in terms of suffering.
Secondary outcomes and resuscitation are a different story, but are obviously not applicable here.
I think I read something about the sub having a CO2 scrubber which would mean their bodies wouldn't feel the lack of oxygen due to what you explained, but I know nothing about this.
This is a good question! Not sure which precise units they had and in what quantity, but given the size of the Titan (no way they can support liquid regenerative system with their size and energy reserve constraints), they would have had canister containing solid CO~2~ adsorbent with a fan (example).
Without the fan, it's not going to be very effective since CO2 has to actually pass over the solid. Passive diffusion is not going to move the same volume of CO2 over the solid even if the solid was removed from the housing. Even if they didn't run out of battery, The solid has a maximum capacity - about 7.5 kg for the unit linked above. Even with reserve capacity, an average human exhales ~0.97 kg of CO~2~ per day.
O~2~ to CO~2~ exchange via respiration is mole for mole (you do lose a little mass in carbon and water just by breathing!). Atmospheric CO~2~ is 0.041% (410 ppm) and O~2~ is a hair under 21% and that's the standard to which life support systems are held. Humans lose consciousness at around 3.7% oxygen, but experience hypercapnia at >6% CO~2~. (Physiology nerds - I converted from the partial pressures in mmHg to % of 1 atm for comprehension)
So in this hypothetical scenario, hypercapnia would definitely precede loss of consciousness due to anoxia.
Okay, so I am gonna check those two off my lists of ways I want to die.