What effects did the different air pressure and air composition in certain spacecraft have on astronauts?

Question

The Mercury and Gemini spacecraft, and in space the Apollo spacecraft, didn't pressurize their cockpits to 1 atm with Earth-like atmospheric composition; instead, they used pure oxygen at much lower pressure levels. This allowed for lighter spacecraft. Now, since the astronauts were breathing pure oxygen in lower total pressure but actually more oxygen than on Earth, I wonder whether that was noticeable for the astronauts – and if so, what effects did this markedly un-Earthlike atmosphere have on them? Reportedly, more oxygen would make people more happy. Did the astronauts feel happier in the oxygen-rich air, for instance?

Cabin pressures of American spacecraft using pure O₂ cabin atmosphere:

• Mercury: 5.5 psi / 0.38 bar
• Gemini: 5.3 psi / 0.37 bar
• Apollo: 5 psi / 0.34 bar

Source

Answer 1

The partial pressure of oxygen (ppO₂) should be higher than about 0.16 bar and lower than about 0.4 to 0.5 bar for longer exposition of some days up to a week. So breathing pure oxygen for a week is possible when the total pressure is not above 0.4 bar.

See also my answer to the question How long were the Apollo astronauts allowed to breathe 100% oxygen at 1 atmosphere continuously?

You may breathe air (oxygen content 21%) at a total pressure of 1 bar to get a partial oxygen pressure of 0.21 bar or you can breathe pure oxygen at a total pressure of 0.21 bar. In both cases the partial oxygen pressure is 0.21 bar.

A pressurized cabin of a passenger aircraft is operated at a total pressure of about 0.75 bar and fresh air, the ppO₂ is 0.16 bar. The pilots do their work breathing the same air as the passengers.

Breathing pure oxygen from a mask feels exactly the same as compressed air from a scuba tank with a regulator.

When a scuba diver breathes pure air at a depth of 20m and a total pressure of 3 bar (1 bar from the air above the water and 2 bar from the water) the partial pressure of oxygen is 0.63 bar. It feels the same as just below surface. For some hours the partial oxygen pressure may be higher than the 0.4 bar without problems.

During the experiments Precontinent of Cousteau divers lived in underwater houses at a depth of 11 m breathing compressed air for up to 4 weeks. The ppO₂ was 0.44 bar. These experiments were done during the early sixties years before the Apollo moon landing.