Tech level: modern - like
Atmospheric pressure at local sea level:
- Oxygen ~30 kPa
- Nitrogen ~180 kPa
(So roughly counting 1.5 times more oxygen, 2.25 times more nitrogen)
Technically speaking such atmosphere counts as breathable, sort of. On the other hand atmosphere that we have on Earth at altitude of 4000 m also counts as breathable but not necessary convenient for the most of people.
Would it be worthwhile to place on such planet main human settlement in mountains, to provide them with slightly more Earth-like conditions, where they evolved?
Yes, I know it's a nightmare for logistics and infrastructure.
Yes, I know, I'm realistically limited to something like 3000 m (in RL highest dam built is at 3200 m; with altitude it becomes colder so there would be something like 18 degrees cooler than at surface), so I can get maybe 2/3 of sea level atmospheric pressure.
By worthwhile I mean that extra costs should be reclaimed in higher productivity, lower medical expenditures or at least higher general wellbeing.
- In RL, even though humans can unquestionably tolerate more, passenger jets are pressurised to keep pressure equivalent to 2000-3000 m, which actually puts extra strain on their frame.
- I'm not worried about oxygen, but whether too much of nitrogen would cause some discomfort. (Not nitrogen anaesthesia, just discomfort, dizziness or slightly impaired work efficiency)
- Logan R. Kearsley has a good point, I should be worried about excessive amount of oxygen and newborn retina damage