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Some space suits are designed to provide an atmosphere of 100% oxygen but at a reduced air pressure about 20% that of sea level.

Is breathing in such a suit uncomfortable? It seems like having such a reduced mass/pressure of air might feel strange or uncomfortable, despite having all the oxygen necessary. Do we have record of any astronauts commenting on this?

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    $\begingroup$ Breathing at higher pressure (up to 5 bar) does not feel very different in personal experience. $\endgroup$
    – Uwe
    Sep 6, 2020 at 17:02
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    $\begingroup$ I also wonder how the process of depressurization feels. Stupid 10mbar when driving down a steep hill makes one's ears feel stuffed and pop. I imagine going 70 times that much can't be very comfortable. $\endgroup$
    – SF.
    Sep 6, 2020 at 17:54
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    $\begingroup$ @SF. With some experience and training as well as healthy ears pressure changes during scuba diving are no problem. You should be able to equalize your ears easily. Much more than 10 mbar difference. $\endgroup$
    – Uwe
    Sep 6, 2020 at 18:16
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    $\begingroup$ Former astronaut Tom Jones gives a very detailed story on the sensations of doing an EVA in his book Skywalking. I quoted some excerpts from it in this answer about sounds during EVA space.stackexchange.com/a/44285/6944 He does not mention any effects of the lower pressure except how it made his voice sound. $\endgroup$
    – Organic Marble
    Sep 6, 2020 at 18:45
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    $\begingroup$ @SF Of course, driving down a steep hill presents a pressurization, vice a depress, event... $\endgroup$
    – Digger
    Oct 6, 2020 at 14:29

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Respiratory gasses interact with the respiratory system through their partial pressures.

Room air is 21% O2 @ 760mmHg, or a partial pressure of 160mmHg. So, at first glance, it would seem that breathing 100% O2 at 160mmHg (3.15psi) would be physiologically identical. However, the lungs are at body temperature and wet inside so they are filled with water vapor at the partial pressure of water for that temperature (60mmHg). If you breathe room air at 760mmHg (ppO2 = 160), it gets diluted with water vapor so you end up with ppO2 of 100mmHg in your lungs.

This water vapor effect is what puts a limit on breathing pure O2 in non-pressurized aircraft.

If you want your space suit to provide the same arterial ppO2 as atmospheric room air, you need slightly higher pressure… about 220mmHg (4.25psi).

There is the added issue of “dead space” and re-breathing exhaled CO2, so fudge that number up a bit more. 5.0psi is a round nice number, and happens to be what the 100%O2 atmosphere was in Apollo capsules.

Your respiratory rate is driven by blood ppCO2, not ppO2. If your breathing gas supply was switched to 100% N2, you would have only a slight sensation of something wrong (like you needed to yawn) just before you passed out. (see inert gas asphyxiation) Then you would die. So, as long as the suit’s CO2 scrubber is working, I think the astronaut would be oblivious to low level of ppO2. I suspect a slow suit leak down to vacuum would go unnoticed from a respiratory perspective. But the suit would become delightfully mobile with the lower pressure.

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  • $\begingroup$ Your calculation of pure oxygen at 220 mm Hg is wrong. We have to compare air and pure oxygen within the lungs together with a partial pressure of 60 mm Hg of water vapor. For air with 21 % oxygen at 760 mm Hg ambient pressure we have 147 mm Hg oxygen, 553 mm Hg nitrogen and 60 mm Hg water vapor. So we need 147 mm Hg of pure oxygen and 60 mm Hg of water vapor at an ambient pressure of 207 mm Hg. If you take 21 % of 760 mm Hg, you get 159.6 mm Hg, adding 60 mm Hg results in 219.6 mm Hg. $\endgroup$
    – Uwe
    Oct 27, 2021 at 0:41
  • $\begingroup$ as mentioned here it's important to add supporting links/sources to assertions of facts in Stack Exchange answers. I've added one to get you started, is it possible to add one or two more? Without them, how can future readers tell if this information is true or false? Thanks! $\endgroup$
    – uhoh
    Oct 27, 2021 at 0:51
  • $\begingroup$ You're right. I'm wrong. There are many factors I didn't account for. Like room air is 1% Argon and it has CO2 and water vapor in it. And the respiratory track has dead space which dilutes inhaled gasses. In respiratory physiology, 100mmHg is used for alveolar ppO2. It's a nice number to work with. But to answer the question "What does it feel like to breathe 100% O2 at low pressure?" is, "It feels like nothing." That's what kills people using malfunctioning rebreathers. $\endgroup$
    – Woody
    Oct 27, 2021 at 0:57
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    $\begingroup$ US EVA suits run a pure O2 atmosphere at less than 5 psia. Also, this doesn't answer the question that was asked - how does it feel. $\endgroup$
    – Organic Marble
    Oct 27, 2021 at 1:19

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