Why is breathing pure oxygen at 1 atmosphere toxic, but breathing pure oxygen at 1/3rd atmosphere safe for astronauts?
In: Biology
To understand this you need to understand the concept of partial pressure and a “partial atmosphere” and the fact that oxygen toxicity is not a product of the ratio of oxygen but the partial pressure of oxygen.
The earth’s atmosphere has a pressure of 1 atmosphere, and is composed of 78% nitrogen, 21% oxygen, 1% argon, and a bit of other stuff. What this means is that there is 0.78 partial atmospheres of nitrogen, 0.21 partial atmospheres of oxygen, etc. These are called “partial atmospheres”
Meanwhile, the astronauts were breathing 0.3 partial atmospheres of oxygen, which also happened to be their total atmosphere at the time.
Oxygen doesn’t become toxic until over 0.5 partial atmospheres. You can exceed this by just increasing the pressure of the ordinary earth atmosphere. Compressed air at 3 atmospheres of pressure is just as toxic as pure oxygen 0.6 atmospheres of pressure. This a reason why scuba tanks are not just filled with compressed air (besides the nitrogen problem) and gas mixes intended for deeper dives have lower ratios of oxygen
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