<p>The main reasons the atmospheric oxygen remains constant are:</p>

<ol>
<li>The sheer quantity of it. Not even a large forest fire will
measurably deplete it. Most of it is fossil oxygen created many
millions of years ago by photosynthetic organisms.</li>
<li>It is constantly being replenished by photosynthetic organisms,
mainly in the ocean.</li>
<li>Measurements of the percentage of oxygen in the atmosphere are a
modern phenomenon. There are no accurate measurements of atmospheric
oxygen in previous geological periods, but we know the percentage
has not always been 21 percent.</li>
</ol>

CC BY-SA 4.0
2019-09-26T17:20:20.297Z
I can't speak to their accuracy, but there are measurments/estimates of atmospheric oxygen over geologic times, e.g. https://en.wikipedia.org/wiki/Geological_history_of_oxygen
jamesqf
CC BY-SA 4.0
2019-09-27T10:17:55.603Z
That's why I included the word "accurate"  Indirect measurements for many millions or billions of years ago are hardly more than educated guesses, and subject to error.. Those for the last 100,000 years or so are a bit more accurate
Michael Walsby
CC BY-SA 4.0
2019-09-25T10:48:57.980Z
2019-09-25T17:11:46.097Z
2019-09-25T17:11:46.097Z
Michael Walsby
<p>I've always wondering why the oxygen in the atmosphere remains so perfectly at the level required for air-breathing life. What causes it to remain so perfect?</p>

<p>Also, a side question, why aren't their pressure bubbles with higher or lower levels of oxygen that could drift through a town and make it hard/impossible to breathe?</p>

CC BY-SA 4.0
2019-09-25T04:28:28.550Z
Re "perfectly at the leve;", you have things backwards.  Air-breathing life can adapt to a wide range of oxygen levels.  Consider for instance that the partial pressure of oxygen decreases as the elevation increases, yet people & other animals live quite happily at elevations well above sea level.  And can scuba dive using a normal air mix to ~40 meters or so.  (Staying long at depths causes problems, but the first ones encountered are due to nitrogen, not oxygen.)
jamesqf
<atmosphere><oxygen>
Why does atmospheric oxygen remain so "constant"?
Chris
# Why does atmospheric oxygen remain so "constant"?

(Asked by: username_0 on 2019-09-24)

I've always wondering why the oxygen in the atmosphere remains so perfectly at the level required for air-breathing life. What causes it to remain so perfect?

Also, a side question, why aren't their pressure bubbles with higher or lower levels of oxygen that could drift through a town and make it hard/impossible to breathe?

**Tags:** atmosphere, oxygen

> **Comments:**
> **username_1**: Re "perfectly at the leve;", you have things backwards.  Air-breathing life can adapt to a wide range of oxygen levels.  Consider for instance that the partial pressure of oxygen decreases as the elevation increases, yet people & other animals live quite happily at elevations well above sea level.  And can scuba dive using a normal air mix to ~40 meters or so.  (Staying long at depths causes problems, but the first ones encountered are due to nitrogen, not oxygen.)  

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## 1 answers

### Answer by username_2 on 2019-09-25. Score: 1

The main reasons the atmospheric oxygen remains constant are:

1. The sheer quantity of it. Not even a large forest fire will
   measurably deplete it. Most of it is fossil oxygen created many
   millions of years ago by photosynthetic organisms.
2. It is constantly being replenished by photosynthetic organisms,
   mainly in the ocean.
3. Measurements of the percentage of oxygen in the atmosphere are a
   modern phenomenon. There are no accurate measurements of atmospheric
   oxygen in previous geological periods, but we know the percentage
   has not always been 21 percent.

> **Comments:**
> **username_1**: I can't speak to their accuracy, but there are measurments/estimates of atmospheric oxygen over geologic times, e.g. https://en.wikipedia.org/wiki/Geological_history_of_oxygen  
> **username_2**: That's why I included the word "accurate"  Indirect measurements for many millions or billions of years ago are hardly more than educated guesses, and subject to error.. Those for the last 100,000 years or so are a bit more accurate  

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