High Altitude Airfields
Years ago I lived in Quito, Ecuador (SEQM field elevation 9228 MSL) and my third floor (no elevator) apartment was on a hillside around 10,200'. Played tennis at this elevation too. Was flying unpressurized airplanes and helicopters in this area, as well as our base in La Paz, Bolivia (SLLP at 13,323 MSL). One does get acclimated to the low oxygen levels.