Long duration flight with lower atmospheric pressure
Skylab had 5.0 psi as atmopsheric pressure, and other missions had different pressures.
Skylab only used 70% as oxygen (even less than Apollo). But that would still be more oxygen than today (21% out of 14.7 psi).
Schmitt would like Moon and Mars expeditions to return back to the lower pressures....Never heard of detrimental effects aboard Skylab. Maybe it's better not to use too much water for cleaning, though.