I don’t know medical science. I am hypersensitive to monitoring of oxygen levels in the workplace as I have been involved in near fatalities as well as regulatory blind compliance.
In any case, normal O2 partial pressure is 20.9% and OSHA alarms trigger at 19.5% (hazardous) and 16% is dangerous for human life.
Oxygen Depletion Alarm: Monitoring Oxygen Deficiency in Enclosed Spaces
| CO2Meter.com
Furthermore, human processing of O2 has many complicated factors which I do not understand.
Partial pressure of oxygen in the human body: a general review
In any case for those who know more (or care more), it seems plausible that wearing a mask, especially during rapidly changing conditions like doing a heavy set, might be a health hazard.
I do not wear a mask when I lift.