i do know that the oxygen ratio can be a factor in cities like Denver so that the Valjean has trouble with pitch and breath in the opening night of a production of Les Mis. so much so, that it throws him off for the entire performance. *g*

it also affects athletes. there are athletes that train in higher altitudes in the Rockies where there is lower oxygen so that when they travel to low altitudes, they have PLENTY of oxygen and therefore plenty of stamina.