A Path to Steam
Every character has a temperature “comfort zone” within which he suffers no ill effects (such as FP or HP loss) due to heat or cold. For ordinary humans, this zone is 55°wide and falls between 35° and 90°. For nonhumans, the zone can be centered anywhere,
but this is a 0-point feature for a zone no larger than 55°. A larger zone is an advantage. Each level of Temperature Tolerance adds HT degrees to your comfort zone, distributed in any way you wish between the “cold” and “hot” ends of the zone.
Temperature Tolerance confers no special resistance to attacks by fire or ice unless the only damage is a result of a rise or fall in the ambient temperature. In particular, it cannot help you if your body temperature is being manipulated.
In a realistic campaign, the GM should limit normal humans to Temperature Tolerance 1 or 2. However, high levels of this trait are likely for nonhumans with fur or a heavy layer of fat.