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At what atmospheric pressure, assuming an earth-like atmospheric composition, do humans start to feel uncomfortable? I am not asking for the max survivable pressure, but the point where long term human comfort is affected. I know at lower atmospheric pressures people have trouble breathing and doing exercise, and apparently even fertility rates are affected, but what's the equivalent for the opposite?

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    $\begingroup$ "feel uncomfortable" - is hard to quantify. But the pressure itself can increase (very slowly) to 100 atmospheres or more. The main question is what mixture of gases or oxygen-rich liquid you could breathe at this pressure. Normal air becomes toxic at 7-8 atmospheres. So, in your case, it is not a question of 'comfort under pressure'. It is a question of being comfortable after being drowned in oxygen-rich liquid. $\endgroup$ Feb 10, 2023 at 4:25
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    $\begingroup$ Among other issues with saturation diving is messing with the sense of smell and the fact that food cools off fast due heavier atmosphere, a question if these count as 'comfort' issues. There are also normally humidity issues that impact both comfort and health but unsure if that is pressure, or a side effect of these pressures normally being encountered underwater. $\endgroup$ Feb 10, 2023 at 6:17
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    $\begingroup$ Scuba divers (saturation or not) seem to have no "comfort" issues going to quite high pressures, but they do need to breathe special gas mixes. $\endgroup$
    – user253751
    Feb 10, 2023 at 15:27
  • $\begingroup$ @TheMatrixEquation-balance Normal air should not be used with a pressure of more than about 4 to 5 bar, especially when used for more than some hours. A nitrogen narcosis is possible. $\endgroup$
    – Uwe
    Feb 10, 2023 at 18:43
  • $\begingroup$ @Uwe - so what did I do wrong again? I did write: "Normal air becomes toxic at 7-8 atmospheres" $\endgroup$ Feb 10, 2023 at 19:46

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I can’t answer your question with a specific pressure, since symptoms depend on the individual, the rate of descent, time at depth and lifetime exposure to high pressure.

Symptoms from high pressure can be due to either the breathing gasses or to the pressure itself.

Gas toxicity:

Most physiologic and toxic effects of breathing gasses are determined by their partial pressures. Obviously, the sum of all partial pressures must equal the ambient pressure. So when the partial pressure limit of a gas is reached, an inert gas must be added.

  1. Oxygen toxicity. High concentrations of oxygen, up to 100% at 2 atmospheres (partial pressure of 30psi), can be tolerated for up to a few days. But, as a rule, 100% oxygen cannot be breathed at 1 atmosphere for more than a week without risking inflammation of the lungs. In long term hyperbaric habitats, oxygen partial pressure is usually limited to 6 psi (sea level air is 3.15psi). Toxicity symptoms are related to inflammation of lung tissue

  2. Nitrogen acts as a general anesthetic (nitrogen narcosis) at partial pressures over about 46psi (sea water equivalent depth of 115 ft breathing compressed air). Mental performance and judgment deteriorate with increasing partial pressure until unconsciousness occurs.

  3. Helium is added when the total combined tolerated partial pressure of both oxygen and nitrogen is less than the ambient pressure. However, Helium is excitatory for the central nervous system (see high pressure nervous syndrome below) so sometimes the partial pressure of nitrogen must be increased to provide a “general anesthetic” effect to counteract this excitatory tendency. Helium also causes problems with thermoregulation.

Pressure symptoms

  1. Compression arthralgia (joint pain) occurs during compression (the period of increasing pressure). It tends to improve with time at depth or decompression. It can limit work and depth. There is not a specific depth at which it occurs since it is caused by the change in pressure, not the absolute pressure

  2. Dysbaric osteonecrosis (bone death). This is a problem with long term exposure to high pressure, as seen in professional divers.

  3. High pressure nervous syndrome is seen at depths of over 500 feet (about 15 atmospheres) while breathing helium. It results in tremors, memory lapses, visual disturbances and somnolence.

  4. Thermoregulation. Helium is widely used in saturation diving habitats. The low molecular weight gives high molecular velocity at a given temperature, and therefore rapid conduction of heat (six times that of air). Sometimes dry suits are inflated with Argon (heavy molecular weight, slow heat conduction) to counter the thermoregulation effects of Helium

  5. Breathing resistance is caused by the increased density of breathing gasses at higher pressure. At sufficient depth, the effort of breathing can limit exercise tolerance and cause buildup of carbon dioxide in the blood. Unfortunately, Argon is particularly dense. This limits its usefulness as an inert gas.

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  • $\begingroup$ You are so wrong, partial pressure of oxygen should be lower than 1.4 to 1.6 bar for some hours and lower than 0.5 bar for days. $\endgroup$
    – Uwe
    Feb 10, 2023 at 19:34
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    $\begingroup$ Unfortunately, much of this is simply incorrect. $\endgroup$
    – fectin
    Feb 10, 2023 at 20:17
  • $\begingroup$ @Uwe ... "Symptoms of toxicity typically begin after ∼12–16 h at 1.0 ATA, 8–14 h at 1.5 ATA, and 3–6 h at 2.0 ATA. Symptoms occur earlier at 2.5 and 3.0 ATA, but are milder since exposure time is limited by neurologic oxygen toxicity." ncbi.nlm.nih.gov/pmc/articles/PMC5467109/…. $\endgroup$
    – Woody
    Feb 10, 2023 at 20:28
  • $\begingroup$ @fectin ... Please elaborate. If you submit an answer you will have a format for correcting errors you have found. $\endgroup$
    – Woody
    Feb 10, 2023 at 21:39
  • $\begingroup$ The values for oxygen toxicity are valid for hyperbaric oxygen treatment of patients, they are not valid for divers or astronauts. The risk of loosing consciousness is acceptable only for patients under supervision by medical personnel. $\endgroup$
    – Uwe
    Feb 10, 2023 at 22:09

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