... also I should add.
If anything, we need to limit the amount of oxygen in the air. Just a little bit TOO much and BOOM !!!!!!!!!! We could actually set the SKY on FIRE.
As proof of the danger of too much oxygen, the reading public simply needs to remember back to the Apollo Capsule explosion back in 1967.
One single spark exploded the whole capsule and killed all three astronauts.
Which was DUE to the FACT that the capsule had been pressurized with almost pure oxygen.
Oxygen is such a destructive and volatile element. Starting at 20% of the volume of air, we then must reduce the amount of that that actually gets extracted by our lungs (easily less than half, worse if you smoke).
So, we are talking an extremely small amount of oxygen transferred into our circulatory system.
Yet, that very small amount of oxygen (because it BURNS so easily) supplies the ENERGY for every cell in our body.
O2 is only about 21% of the atmosphere.
The real issue is what can our blood absorb. The surface atmospheric pressure is about 14.7 PSI. The oxygen partial pressure is about 3.1 PSI. Our blood can only absorb O2 in a limited range of partial pressure. Too high you get whats called oxygen toxicity and you will die. To little and you get oxygen starvation.
That is why when you SCUBA dive, beyond a certain depth you have to used mixed gases, not just straight air, in SCUBA tanks because the air pressure from the regulator must increase to match the water pressure as you descend in depth. A mix like HeliOx for deep diving will reduce the O2 partial pressure that has increased due to depth to keep it near the 3.1 PSI range.
Likewise, the US space program used to use 10 PSI air, with a higher percentage of O2, again to try to keep the partial pressure of O2 the same. Not sure what mix the space program uses today. The Russians always used 14.7 PSI air. I think when we started doing joint missions, we also went to straight 14.7 PSI air.