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A little while ago, I asked a question about how habitable a gas planet I created could be. Although I expected that the floating cities idea wasn't the most solid, I was surprised that one of the biggest issues with it was its oxygen rich atmosphere.

I know that oxygen can help the human body in slightly higher concentrations and harm it if it's extremely high. But at what level of oxygen does it become too dangerous for an average human to breath?

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marked as duplicate by James, Hohmannfan, Frostfyre, Vincent, bilbo_pingouin Jun 28 '16 at 19:42

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  • $\begingroup$ Oxygen toxicity occurs at partial pressures above 1.6 atm. Source: en.wikipedia.org/wiki/Oxygen_toxicity $\endgroup$ – ckersch Jun 27 '16 at 22:23
  • $\begingroup$ Even if humans and fauna could evolve to adapt to higher oxygen concentrations, chemistry could not. More oxygen in the atmosphere = more spectacular fires and explosions. $\endgroup$ – Renan Jun 28 '16 at 16:26
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    $\begingroup$ @Renan Considering the fact that it's the setting for a huge sci-fi battle, that's definitely not a bad thing :) $\endgroup$ – Mattias Jun 28 '16 at 18:09
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There are many good studies on oxygen toxicity. One NASA page reviews many studies on the limits. Tobias (1967) came up with the following graph of the limits of the percent of oxygen vs. the total pressure of the environment:

Really, the limits depend on the rest of the atmosphere. For example, at a total pressure of 90 Kpa, you can have at most an oxygen concentration of about 50%, though effects will begin at about 35%. However, note that in atmospheres that are almost entirely oxygen, combustion becomes an issue. See the Apollo 1 disaster.

Duration is also a necessary factor, as this university page explains:

Oxygen toxicity limits in human beings. The two areas most affected are the CNS and the lungs. The occurrence of toxicity depends upon both the inspired oxygen pressure (PO2) and the duration of exposure. The safe duration of exposure becomes shorter as the inspired PO2 increases. Below 0.5 atmosphere of inspired oxygen, indefinite exposure appears to be safe; between 0.5 and approximately 2 atmospheres, pulmonary toxicity occurs after prolonged exposures but CNS effects are not detectable; above 2 atmospheres, CNS toxicity appears before pulmonary effects are detectable.

In a gas dwarf, how much of the atmosphere is likely to be oxygen? Here's (a modified) Figure 9 from Lopez & Fortney (2013):

I've boxed in the area between $2M_{\oplus}$ and $5M_{\oplus}$, the area where we're likely to see gas dwarfs, at some likely envelope radii. The atmospheres are likely to be 30%-100% hydrogen and helium, meaning that they can be 70%-0% oxygen (and other heavy gases, which can be just as dangerous if their partial pressures are increased). At 70% oxygen, effects start to appear at total pressures of about 45 kPa; at 0% oxygen, humans can't survive (the minimum should be 10%-20%, depending on the total pressure).

I don't know what the likely total pressures are - which could help us narrow down the range - but it seems like you've got a decent amount of wiggle room, in your gas dwarf scenario.

I assume there would be some atmospheric stratification, as suggested by Ville Nieme, but I don't know for sure.

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    $\begingroup$ Good work. Maybe a mention that if you add pressure and compensate by reducing oxygen and this results in partial pressure of nitrogen or other inert gas increasing, that comes toxic as well. $\endgroup$ – Ville Niemi Jun 27 '16 at 22:42
  • $\begingroup$ @VilleNiemi Done; I also added some other bits. $\endgroup$ – HDE 226868 Jun 27 '16 at 23:14
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    $\begingroup$ Is there any data on whether the atmosphere would become stratified? HE/H are significantly lighter than oxygen, so they wouldn't really mix? $\endgroup$ – Ville Niemi Jun 27 '16 at 23:29
  • $\begingroup$ @VilleNiemi I don't have data on that (not that it doesn't exist, of course), but I would assume that some stratification would take place. $\endgroup$ – HDE 226868 Jun 28 '16 at 21:14
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Oxygen gas ($O_2$) can be toxic at elevated partial pressures, leading to convulsions and other health problems. Oxygen toxicity usually begins to occur at partial pressures more than 50 kilopascals (kPa), equal to about 50% oxygen composition at standard pressure or 2.5 times the normal sea-level $O_2$ partial pressure of about 21 kPa. This is not a problem except for patients on mechanical ventilators, since gas supplied through oxygen masks in medical applications is typically composed of only 30%–50% $O_2$ by volume (about 30 kPa at standard pressure). (although this figure also is subject to wide variation, depending on type of mask).

Breathing pure $O_2$ in space applications, such as in some modern space suits, or in early spacecraft such as Apollo, causes no damage due to the low total pressures used.In the case of spacesuits, the $O_2$ partial pressure in the breathing gas is, in general, about 30 kPa (1.4 times normal), and the resulting $O_2$ partial pressure in the astronaut's arterial blood is only marginally more than normal sea-level $O_2$ partial pressure

Source: Wikipedia

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This is slightly complicated. The measure that matters is the "partial pressure" of oxygen: the amount of the atmospheric pressure that one is under which is made up of oxygen. There's no great health risk at one atmosphere's worth of pure oxygen, although trouble starts fairly soon above that. The Wikipedia article on Oxygen Toxicity seems pretty comprehensive.

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