I am thinking of a scenario in which the protagonist travels to a world relatively similar to the earth, but with certain differences with respect to the atmosphere, specifically its atmospheric pressure and its proportion of oxygen. What would make that despite being a breathable atmosphere and able to sustain human life, it shows differences. For example, I am thinking that due to the sudden change of atmospheric pressure the protagonist will get dizzy for some time, although for obvious reasons without reaching a deadly scale.

Anyway, I would like help to build an atmosphere capable of sustaining human life and a certain level of development, but still present remarkable differences with respect to that of our world. With what I was reading from time to time I decided that the atmosphere would have 1.15 atm and its oxygen ratio would be 23% having its limit at 25%, however I do not know if this model would really be viable or if it can push it a bit more.

I want to emphasize, I AM NOT DOING VARIOUS QUESTIONS, I only ask for a single answer based on the pressure and the proportion of oxygen. By the way, before you say this is a duplicate, let me clarify a couple of things.

  • When we speak of hypothetical worlds where the atmospheric pressure is greater than that of the earth, none takes into account the narcosis produced by inert gases, which in the case of nitrogen (the most abundant gas in our air) it´s 2atm. So... unless you want the population to be drugged all the time, it would be best to take this into consideration.
  • When we talk about the maximum proportion of oxygen that humans can
    breathe, None takes into account that in order for intelligent life
    to reach, at least, to be able to achieve a pre-industrial level of
    development, it must be allow the manipulation of certain elements.
    Among them the most vital and indispensable is fire. It is useless
    for humans to breathe X proportion of oxygen, if at the moment we try to light a bonfire, we explode in a fireball. Therefore, it is
    necessary to consider the capacity to be able to manage the fire in
    an atmosphere enriched in oxygen.
  • On the other hand, it must be taken into account that air when subjected to other conditions of atmospheric pressure may be more or less tolerable in the long term. Subject that I do not really dominate much.
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    $\begingroup$ en.wikipedia.org/wiki/Oxygen_toxicity $\endgroup$
    – RonJohn
    Apr 3 '18 at 3:40
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    $\begingroup$ There are places on Earth, for example on the shores of the Dead Sea, where normal atmospheric pressure is 1.05 bar. So the 1.15 bar and 25% oxygen on this hypothetical world is so close to normal Earth conditions that I seriously doubt that anybody would be able to tell the difference without resorting to actual scientific measurements; fires will start just a little easier, flight will be just a little easier to achieve; air resistance will be just a little stronger so aircraft will use just a little more fuel, stuff like that. There will be no oxygen toxicity or nitrogen narcosis. $\endgroup$
    – AlexP
    Apr 3 '18 at 3:55
  • $\begingroup$ Aside from the question title, I don't see a question here. Plenty of discussion about atmospheric qualities and what answers need to take into account, but I don't know what you want to know. Voting to put on hold as unclear what you're asking. $\endgroup$
    – Frostfyre
    Apr 3 '18 at 12:36

So when we're discussing Oxygen Toxicity, it's important to note that we keep the terms correct and for that reason, we're going to talk about partial pressure. That is to say, the part of the pressure that relates to O2.

In other words; if at sea level the atmospheric pressure is 1 bar, and O2 comprises 21% of the atmosphere, then the partial pressure of O2 for our purposes is 0.21 bar.

This is an important consideration as when we get to the point of considering O2 as a problem, we're not considering it AND something else or O2 in combination; we're purely talking about O2 as a damaging agent and tests have proven that it's the partial pressure which is concerning, NOT the dilution rate of the O2 in atmosphere.

That said; the already provided link will show that the first thing you have to be concerned about is your lungs. High O2 partial pressures will also fry your retinas and cause neural damage, but it's your lungs which will suffer first. Anything above a PP of 0.5 bar is a problem.

When the Apollo astronauts flew to the moon and back, they did so in a pure O2 environment. That was very dangerous in terms of sparks igniting the atmosphere in the cabin, but for the pilots themselves they suffered no real issues with it. Why? Because they dialled down the atmospheric pressure. By dialing down the atmospheric density to (say) 0.4 atmospheres but making it pure O2, you put less pressure differential between the inside of your ship and the vacuum of space and you carry less weight into space for breathing gasses over the duration of the mission.

Getting rid of CO2 is actually the limiting factor for cramped conditions over time; anything more than about 1% CO2 causes more problems than does O2 toxicity in the short term. So; provided you can scrub the CO2 out of your O2 environment, your ship's walls can be thinner, the breathable air lighter; there's a lot of reasons to do that.

But in your case, you're talking about increasing density. So; provided the density of gas increases; the best solution is to decrease the proportion of O2 equivalently and you're good. At worst, don't let the differences get to more than double a conventional Earth-like atmospheric distribution.

Example; going to 1.15 ATMs but keeping the O2 percentage should be fine. Going to 1.5 ATMs and keepting the O2 percentage is alright, but probably not ideal. Going to 2 ATMs but reducing O2 to (say) 15% (meaning 0.3 bar PP) is fine.

Of course, there are some caveats.

This is based purely on short term testing. We haven't seen what happens to someone who lives a lifetime in 0.5 bar PP O2 and my guess is that over time some long term factors could come into play. Generally speaking O2 is a key chemical in releasing energy in our bodies but it also appears to play a critical role in our ageing process. This is why antioxidants are so popular as a health supplement. It's possible that at 0.5 bar, people could operate with more energy, be more agile, faster runners et al but eventually live a shorter lifespan as a result. We just don't know yet.

Also, what I've been describing is the effect of O2 PP on humans. If you have creatures that have evolved in higher PP levels of O2, then you would expect them to have metabolisms and physiological structures (like perhaps smaller lungs) to cater for this change.

Also, if you have a natural fear of insects, then you don't want to live on such a world. Insects don't have lungs and rely on O2 absorption through their carapaces saturate their bodies with O2, meaning that the PP of O2 places a limit on how big they can grow. More O2 in the atmosphere, the larger they can grow.

Finally, if you keep the atmospheric composition similar to Earth's but just increase the pressure, swapping out excess O2 PP for more Nitrogen, you run into a completely different problem. Once you reach about 3 ATMs, the nitrogen PP is sufficient to trigger a narcotic effect. Continuing to increase it ALSO leads to toxicity. Modern scuba divers know this and often have to dive deeply with special air mixes that reduce both O2 and N2 so that neither have these effects. It should also be noted that such a high pressure means that your CO2 scrubbing systems would have to be VERY efficient; preferably a very high concentration of vegetation to animal life on such a high density atmosphere planet. Why? Well that same issue with PP also means that at 3 ATMs, CO2 concentrations start to become a problem at around 0.35%.

So; if you're designing a dense atmosphere, O2 is certainly a consideration, but it's not the only one. Your best starting position is researching scuba air mixes for deep dives, then using that as a starting point for your density calculations in terms of PP and what other gasses you could use to thin the air out a little, so to speak.

  • $\begingroup$ In fact, the presence of giant insects is one of the things I wanted to introduce into history. On the other hand, a difference in partial pressure of 0.15 would be sufficiently sudden to cause dizziness and nausea, or would it not be noticed? $\endgroup$
    – JAMS
    Apr 3 '18 at 4:06
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    $\begingroup$ @JAMS like AlexP's comment above, I'm pretty sure that just increasing the pressure to 1.15 ATM isn't going to cause any significant difference at all. I've been down to 30m diving, which is similar to 4 bar using a normal air mix (but admittedly for a short period) with no deleterious effects. 1.5 ATM, normal air mix would give an O2 PP of around 0.33 and that could result in significantly larger insects based on what research I've done previously. $\endgroup$
    – Tim B II
    Apr 3 '18 at 4:15
  • $\begingroup$ For there I read that at higher atmospheric pressure the weight of things was less noticeable. That's right? $\endgroup$
    – JAMS
    Apr 3 '18 at 4:47
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    $\begingroup$ @JAMS sort of; water is very dense and as such there are things that are more neutrally buoyant in water than in (say) air. But, if you're pushing directly up for instance, remember that the higher density means more resistance pressure working against you. You wouldn't be able to throw a ball as far (for instance) but a plane's wing would theoretically work better, which it would need to because the plane's propeller will now need more energy to push through the denser air. What I'm trying to say is think in terms of mass, not weight. $\endgroup$
    – Tim B II
    Apr 3 '18 at 5:15
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    $\begingroup$ Ah. That goes a little outside my 'off the top of my head' knowledge base but I do know that different atmospheric gases (including water) can change between gas and liquid at different temperatures at different pressures. Certainly, the amount of momentum in winds on such a planet would be cause for concern and even a relatively gentle breeze could knock you off your feet at the right atmospheric pressure. $\endgroup$
    – Tim B II
    Apr 3 '18 at 7:37

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