My personal experience is that breathing is a bit harder due to the resistance of the mask and also exhaled Co2 again being inhaled while running. The other symptom that i have experienced is dry eyes due to hot exhaled air going via the edges of the mask into my eyes. But this is a specific experience not a generic one.Why unhealthy?
That is the arguments that are being used here in the USA. But I have not heard the reasoning.
If anything it will increase the hematocrit levels.