# Is an atmosphere's oxygen partial pressure or concentration more biologically significant?

I'm confused about the relationship between partial pressure and concentration of a gas, in relation to the biology of respiration. If I have a planet with high atmospheric pressure - maybe like 10 atm - but the O2 concentration is lower, maybe around 10%, what effect would this have on oxygen-breathing life? On Earth, oxygen makes up 21% of our atmosphere, with a partial pressure of 0.21 atm. In a 10 atm atmosphere, with oxygen at 10%, this would be 1 atm, which is higher than on Earth, obviously. Would an atmosphere like this provide higher or lower access to oxygen than on Earth? I guess what I'm wondering is, is partial pressure or concentration of oxygen more biologically significant?

• Pressure and concentration are certainly related, but they also deal with very different aspects of physiology. A pure-oxygen low-pressure environment may provide enough oxygen for the body to live, but there are a host of problems that come from living at very low pressures. Likewise, the pressure can be high enough to avoid all those problems, but 100% oxygen at that point is toxic. So when you ask which is more significant, the answer is, "it's a curve. The curve is significant." You need the right amount of oxy at each point along the range of acceptable pressures.
– JBH
Oct 4, 2022 at 20:09
• BTW, could you explain why it's important to know a detail to that extreme? The likelihood of your story being read by people who would recognize a presented percentage as impossible is close to zero and human history has proven that if you write a good story people will ignore inconsistent or missing details while if you write a bad one all the details in the world won't save it. So if you don't mind, why do you need so much detail?
– JBH
Oct 4, 2022 at 20:12
• The Apollo missions famously used 100% oxygen at 0.34 atm (5 psi). There were no adverse effects on the astronauts. (Gases mix freely, and each component of the mixture behaves pretty much as if the others were not there; there is little difference between 100% oxygen at 1 atm and 10% oxygen at 10 atm -- and 1 atm partial pressure of oxygen is not healthy in the long run.) (But 10 atm is quite high pressure; I would start inquiring about the effects the other gasses at that pressure.) Oct 4, 2022 at 20:25
• @JBH I'm making a fake scientific textbook/artbook, about how life might evolve on planets with different conditions. So it actually is important to me to get the science right. For most readers, it will be more of a coffee table art book, with the main point being the illustrations and the writing serving the aesthetic, giving the book legitimate textbook vibes. But I want the book to work on multiple levels and to be accurate for those interested in the science. So I really want the science to be actually plausible. Oct 5, 2022 at 0:47
• @JBH - I'm literally world-building, though. My project is not factual, it's fictional, but based on scientific principles. I'm designing a fictional planet, just like everyone else here. I really appreciate the help you've given me, but I don't understand why you would make a post to call into question whether or not I should be allowed to use this resource. Most of the time when I get answers here, I use them to give me clearer direction, as ultimately the main resources I'm using are journal articles and directly contacting scientists. Oct 5, 2022 at 18:49

## 2 Answers

Take a look at the Hyperbaric chambers for both medical purposes and treating decompression sickness. Some of the hard chambers go up to 8 atmospheres and include 100% oxygen , but with scheduled air breaks where the oxygen percentage is brought down to 21% to reduce oxygen toxicity.

The Navy dive manual is interesting reading and there is quite a bit of we know this happens but maybe we don’t know why when it comes to oxygen toxicity. As mentioned in the comments a lot of that uncertainty is suspected to be the role of the inert gasses. The tolerance to oxygen is higher when dry than wet for example. Why? Not sure, but you treat wet suits different than dry suits. Take someone off oxygen, and they go into convulsions why? Shrug, it doesn’t seem to be that bad and is like a “vigorous workout”. What is the role of CO2, it matters…. So it is pretty empirical document, but it has lead to a field of hyperbaric medicine. Some claims are probably unrealistic.

Heliox (mixtures of helium and oxygen) are used for two reasons medically there is less resistance when breathing so that helps with breathing difficulty in a medical context. It also prevents nitrogen narcosis or rapture of the deep and the anesthetic nature of inert gas.

Trimix is cheaper, and has enough helium to minimize the chance for narcosis.

So for your planet, it seems that you have some license to have some realistic leeway with the oxygen percentages and the effects on people and how they respond.

With the partial pressure you also have some influence on how flammable things are. But around 21 percent that shouldn’t be that big a problem.

It's the partial pressure that matters for biological processes. There is a minimum pressure of oxygen you need to live. There is a somewhat higher total pressure you need to live because at the minimum oxygen pressure you get too much water evaporation in the lungs.

Otherwise, all that matters is toxicity and stability. (You can, for example, breathe perfectly well in 20kPa O2 and 80 kPa H2 but one spark and everything goes boom.) Everything you could breathe has a point where it becomes dangerous. Deep sea diving reaches a depth limit not because of what the pressure does to a person, but what you can breathe.