Thin

Thinner atmospheres tend to cause a nonacclimated creature to have difficulty breathing and become extremely tired. A typical thin atmosphere requires such a creature to succeed at a Fortitude save each hour (DC = 15 +1 per previous check) or become fatigued. The fatigue ends when the creature returns to a normal atmosphere.

Severely thin atmospheres can cause long-term oxygen deprivation to those affected in addition to the effects of a standard thin atmosphere. The first time a creature in a severely thin atmosphere fails its Fortitude save, it must succeed at a DC 25 Fortitude save or take 1 damage to all ability scores. A creature acclimated to high altitude (see Hill and Mountain Terrain on page 397) gains a +4 insight bonus to its saving throw to resist this effect.