To back up what you're saying about CO2 levels – I used to work in a small office with about 40 other devs. In the winter when the windows were closed the room

To back up what you're saying about CO2 levels – I used to work in a small office with about 40 other devs. In the winter when the windows were closed the room was basically sealed off. The only fresh air we'd get was when someone occasionally entered or left the room. I was always sceptical of how useful CO2 monitors are, but I was barely functional in that office. A colleague of mine regularly worked in the corridor because he got such bad headaches while in the office. For me I just got drowsy and couldn't focus. I've never tried to actively increase oxygen, but I do carry a CO2 monitor now and I think if you're going to be doing mentally demanding tasks in a confided space it's useful to think about. Personally I wouldn't want to increase O2 concentrations beyond levels you'd generally find outdoors though... Perhaps if you occasionally want a short burst of focus it wouldn't be too harmful, but I'm guessing its effects will be a bit like caffeine in that if you consume it constantly your body will just adjust to a new baseline and you'll find it hard to cope without it.