Heat deals subdual damage that cannot be recovered until the character gets cooled off (reaches shade, survives until nightfall, gets doused in water, and so forth). Once rendered unconscious through the accumulation of subdual damage, the character begins to take lethal damage at the same rate.
A character in very hot conditions (above 30°C) must make a Fortitude saving throw each hour (DC 15, +1 for each previous check) or take 1d4 points of subdual damage. Characters wearing heavy clothing or armour of any sort take a -4 penalty on their saves, while characters wearing metal armour receive a -8 penalty. (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 at a rate of 1d4 points per hour.
In severe heat (above 40°C), a character must make a Fortitude saving throw every 10 minutes.
A character who takes any subdual damage from heat exposure becomes fatigued. These penalties end when the character recovers the subdual damage he took from the heat.
Engaging in combat while wearing metal armour in hot conditions can also quickly lead to fatigue. Such a character will become fatigued after a number of rounds of combat equal to their Constitution score minus the Armour Penalty of the metal armour. If he continues fighting while fatigued, he will become exhausted after a similar number of rounds. Resting decreases the character's cumulative total by one per round spent resting.
For Example: Nappy has a Constitution of 10 and is wearing a chain hauberk (-2 Armour Penalty). He would become fatigued after fighting for 8 rounds in a hot environment. If he fights for 6 rounds then rests for 2, he can go on to fight another 4 rounds before becoming fatigued.
Later, he has to slog across the open desert for an hour under the blazing hot sun at 42°C. He doesn't want to suffer heatstroke, so he ditches the armour. He still has to make six Fortitude checks, however, starting at DC 15 and increasing to DC 20 by the hour's end. He will likely suffer subdual damage and become fatigued.
The fringer's Desert Mastery talent can reduce or avoid many of these penalties.
In the Deadlands, one overriding consideration will almost certainly be the supply of water. These dehydration rules are intended for extreme situations and should only be enforced when a lack of water could be life threatening.
A character's water consumption needs depends upon his level of activity, size, and encumbrance. An active character (hard exertion, walking, riding, etc.) needs 4 litres of water per day. An inactive character (sitting, resting, or sleeping, etc.), needs 2 litres of water per day. Small characters need half as much, while large characters require double. A character who is heavily encumbered or wearing a metal armor requires twice as much water each day to avoid dehydration. If the character is in the shade during the entire day, he only needs half the amount of water dictated by his size and activity.
A character can go without water for 1 day plus a number of hours equal to his Constitution score. After this time, the character must make a Constitution check each hour (DC 10, +1 for each previous check) or become dehydrated and sustain 1d6 points of subdual damage. Characters who have taken subdual damage from lack of water are considered fatigued. Subdual damage from thirst cannot be recovered until the character gets water, as needed - not even psychic healing that restores hit points heals this damage.
A character can rehydrate by drinking his full allotment of water through the course of one day. Many common beverages such as wine, beer, ale, and fruit juices can supplement a character's water intake-the quantities per day remain the same. In times of desperation, players may suggest more outlandish liquids to stave off dehydration: honey, tree sap, even the blood of fallen monsters. Generally, none of these are suitable substitutes.
Most of the inhabitants of Deadlands choose to travel after dark instead of during the day. At night, the temperatures in all types of terrain drop significantly, thus avoiding most heat effects, though moisture is still at a premium. If characters decide to travel by night, they gain the benefit of working in shade (half water consumption).
The drawback to such plans is that good rest under the blistering sun of the day is difficult. Characters who are travelling by night must seek shelter during their daytime rest periods. (The fringer's Shelter class ability can help with this.) Rock outcroppings or caves will suffice, as will tents or other make-shift buildings. If such shelter cannot be located, each character must make a Fort save (DC 15) in order to rest well. Those who fail will sleep fitfully and can not recover hit points or ISP.