Low-pressure/high-O2 atmospheres

Here's a Usenet archive which delves into some of these issues: http://yarchive.net/space/science/breathing_atmospheres.html Henry Spencersays: Skylab did not use pure O2 -- it ran at 5psi with about 75% oxygen and 25%
nitrogen. Also, there was some reduction of lung capacity observed in
some of the Skylab astronauts, although it was mild and vanished quickly
after return.
I'd assume that reference toreduction of lung capacity was the alveoli issue I'd mentioned. At the end hementions that pressure/O2 percentage can't be considereda straight linear relationship because one has to factor in the presence of CO2 and water vapor in the lungs, which remains about the same. I assume that Skylab was our longest experience with a reduced-pressure/enhanced-O2-percentage atmosphere. Shuttle and ISS use sea-level pressure air. I had trouble finding info on what Mir used to use.
Regards,
Mike Combs