Heat and Cold
Heat and cold deal damage that cannot be recovered until the character counteracts or escapes the inclement temperature. As soon as the character suffers any damage from heat or cold, he or she is considered fatigued.
A character not properly equipped to counteract the heat or cold must attempt a Fortitude saving throw each hour (DC 15, +1 for each previous check). Failure means that the character loses 1d4 hit points. Heavy clothing or armor provides a –4 penalty on saves against heat but grants a +4 equipment bonus on saves against cold. A character who succeeds at a Survival check (DC 15) gains a +4 competence bonus on the save (see the Survival skill).
Searing heat or bitter cold (desert or arctic conditions) forces a character to make a Fortitude save every 10 minutes. Failure means that the character loses 1d6 hit points. Appropriate clothing and successful use of the Survival skill can modify the save, as noted above.