30,000 ft to 60,000

All nonadapted creatures labor to breathe in the thin air at this altitude. Characters must succeed on a Con save each hour (DC 15, +1 per previous check) or gain one level of fatigue. Fatigue cannot be subsided until supplemental oxygen can be administered or enter a lower elevation. Adapted cretures do not suffer from this effect or lower. Mountainborn is a prime example of this.