Well the DM has the final call as always, but here's what I came up with from an adventure I plan on running in the future.
Heat deals nonlethal damage that cannot be recovered until the character gets cooled off... Once rendered unconscious through the accumulation of nonlethal damage, the character begins to take lethal damage at the same rate.
In areas of severe heat (above 110 degrees F)... a character must make a Fortitude save once every 10 minutes (DC 15, +1 for each previous check) or take 1d4 points of nonlethal damage. Characters wearing heavy clothing or armor of any sort take a -4 penalty on their saves. A character with the Survival skill may receive a bonus on this saving throw and may be able to apply this bonus to other characters as well. Characters reduced to unconsciousness begin taking lethal damage (1d4 points for each 10 minute period).
Areas of extreme heat (air temperature over 140 degrees F)... deal lethal damage. The spell endure elements is not sufficient to protect creatures not acclimated to fire. Breathing air in these temperatures deals 1d6 points of damage per minute (no save). In addition, those wearing metal armor or coming into contact with very hot metal are affected as if by a heat metal spell.
Lava or magma deals 2d6 points of damage per round of exposure, except in the case of total immersion (such as when the character falls into the crater of an active volcano), which deals 20d6 points of damage per round. Damage from magma continues for 1d3 rounds after exposure ceases, but this additional damage is only half of that dealt during actual contact (that is, 1d6 or 10d6 points per round).
An immunity or resistance to fire serves as an immunity to lava or magma. However, a creature immune to fire might still drown if completely immersed in lava.
Chris Doyle and Adrian Pommier, Castle Whiterock, pages 488 and 496.
Hope this helps.
Oh Come On! Everyone knows that Lava doesn't kill you! It just turns you into Darth Vader!
