The Air Force Academy Falcon Stadium is at 6,600 feet (just over 2000m) in elevation. The ramp down to the field is full of signs (particularly for visiting teams) warning about hypoxia.
Speaking from personal experience, pushing the percussion pit equipment at a marching band competition, that's not so much a problem going downhill into the stadium, as in pushing all that equipment back up the hill in existing.


It's partly a psychological ploy. The University of Wyoming has (had?) some similar signs, and claims the highest (NCAA Basketball) court in the land.
That's one tough Marching Band; wish mine was like that..