So my question is pretty much as the title states, what would be the effects on the human body with these atmospheric conditions? More specifically, how would effect the growth of land mammals, reptiles, birds, and vegetation?

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    $\begingroup$ Welcome to Worldbuilding, GuyversLab, while your question is concise. Its brevity makes it puzzling. Does this situation exist on Earth or another planet? Oxygen at high pressure becomes toxic. At 2.5 atmospheres, this is close to the lethal limit. Please expand your question to give more context to enable cogent answers. $\endgroup$
    – a4android
    Jun 19, 2017 at 2:57

1 Answer 1


Normal atmosphere is about 101.325 kPa so $101.325 \text{ kPa} \times 2.5$ would be 253.3125 kPa.

Normal oxygen concentration is 20.946% but you want a 25% more O2 so it would be 26.1825%.

Now we have to calculate the partial pressure of oxygen to know if this is breathable, I don't want to make much calculations so I will only think about the 3 most important gas in our atmosphere (if you want tomorrow I can improve the answer) (with low precision values I mean an error of ~1%):

$$\begin{array}{|c|cc|ccc|c|} \hline & \text{Earth} & \text{Your world} & \text{gr/mol} & \text{Mols} & \text{Fractal Mol} & \text{Partial Pressure} \\ \hline \text{N}_2 & 78.08\% & 73.99\% & 28.0134 & 2.64 & 0.75 & 192.19 \space\text{kPa} \\ \hline \text{O}_2 & 20.95\% & 26.18\% & 31.9988 & 0.81 & 0.23 & 59.53 \space\text{kPa} \\ \hline \text{Ar} & 0.93\% & 0.86\% & 39.948 & 0.02 & 0.006 & 1.58 \space\text{kPa} \\ \hline \textbf{Total} & & & & 3.48 & & \\ \hline \end{array}$$

O2 partial pressure is 59.53 kPa, so... What does this mean? Well... at 50 kPa you enter into oxygen toxicity basically you will die in some hour or maybe days.

All the life of your planet would be faster and have a higher metabolic rate, animals would have problems with oxidation (they will slowly rot) and they will age faster (oxygen free radicals). Also, your insect would be much larger. Insects haven't our air-breathing respiratory system like other animals, they breath through their skin *, they can't be bigger because they won't have enough surface per gramme of insect to breathing. With more O2 insect could have bigger bodies without suffocating.

In this answer I explain better the effects of high and low levels of oxygen.


Insect respiration is accomplished without lungs. Instead, the insect respiratory system uses a system of internal tubes and sacs through which gases either diffuse or are actively pumped, delivering oxygen directly to tissues that need it via their trachea. Since oxygen is delivered directly, the circulatory system is not used to carry oxygen, and is therefore greatly reduced. [...] Air is taken in through openings on the sides of the abdomen called spiracles.
The respiratory system is an important factor that limits the size of insects. As insects get bigger, this type of oxygen transport gets less efficient and thus the heaviest insect currently weighs less than 100 g. However, with increased atmospheric oxygen levels, as happened in the late Paleozoic, larger insects were possible, such as dragonflies with wingspans of more than two feet.

  • $\begingroup$ Insects most certainly have a respiratory system; it is indeed very different from other land animals, in that it is made up of a network of tubes called trachaeas, connected to the exterior air by spiracles, which distribute oxygen directly to the tissues; their haemolymph does not carry oxygen. But you are right in that this kind of system cannot support larger animals unless the partial pressure of oxygen is much higher than on earth. $\endgroup$
    – AlexP
    Jun 19, 2017 at 9:14
  • $\begingroup$ @AlexP I'd be ok with a rewording of "they don't have a respiratory system like other air-breathing animals." Ender correctly simplifies their system to "breathing through their skin" which is the important detail here. $\endgroup$ Jun 19, 2017 at 16:45
  • $\begingroup$ @AlexP I know that they have respiratory system, I put that they haven't because it's more easy for understand to a normal person. $\endgroup$
    – Ender Look
    Jun 19, 2017 at 17:30
  • $\begingroup$ @Draco18s I have already edited my post. sorry if I confused someone. $\endgroup$
    – Ender Look
    Jun 19, 2017 at 17:32
  • $\begingroup$ More "politically correct" than "confusing." No harm done (although Alex could've just edited it himself). :) $\endgroup$ Jun 19, 2017 at 17:35

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