Why is breathing pure oxygen at 1 atmosphere toxic, but breathing pure oxygen at 1/3rd atmosphere safe for astronauts?

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Why is breathing pure oxygen at 1 atmosphere toxic, but breathing pure oxygen at 1/3rd atmosphere safe for astronauts?

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3 Answers

Anonymous 0 Comments

Partial Pressure of Oxygen on Earth is at about 1/5th of an atmosphere.

At an ELI5 perspective, Oxygen is highly reactive — it oxidizes things and reacts with various tissues in your body. Your body is naturally able to handle that at “normal” concentrations for people (i.e. what’s in the air we breathe day-to-day). But, when you increase that concentration, your body can no longer handle it.

Anonymous 0 Comments

It is all about partial pressures.

For example at sea level we have about 20% oxygen. If you breathe compressed air (SCUBA) at 160 feet deep (5 atmospheres) then because of the pressure it is equivalent to breathing pure oxygen at sea level.

1/3rd atmosphere of pure oxygen is like air that is 33% oxygen at sea level.

* all numbers are approximations – do some googling to get more accurate numbers.

Anonymous 0 Comments

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