Rules for Exposure
Characters lacking certain physical necessities accrue 1d6 damage per time unit, as outlined below:
- No Air: 1d6 per minute.
- No Shelter: 1d6 per hour. *
- No Water: 1d6 per day.
- No Food: 1d6 per week.
There is no saving throw for this damage, and it cannot be healed by "cure wounds" magic. Once hit points are lost, a character suffers a -2 fatigue penalty to hit and AC. When the exposure condition ends, hit points are regained at the same rate they were reduced (1d6 per appropriate time unit). This rule should only be applied at an equivalent game time-unit; for example, if wilderness maps and action are scaled to 1 turn = 1 day, then a "no water" condition may be assessed, but travel under a "no air" or "no shelter" condition results in automatic death!
Justification for the Rule
First of all, one might meditate on the oddity that classic D&D lacked any rules for exposure factors, considering how closely and explicitly OD&D was interconnected to the Outdoor Survival game, that being nothing but a simulation of the effects of lacking water and food for a traveler in the wilderness (hopefully, more on that later).
If we do take Outdoor Survival as an example for rules of this nature, then we might think seriously about making a "death track" for each character, which realistically accelerates the degradation effect over time. However, in this author's opinion, nonlinear effects such as those are fundamentally outside the D&D idiom, and should be avoided. For example, it would short-circuit the supernatural endurance of high-level D&D characters (as modeled by hit points), and it would require a new tracking record at the table for every PC and NPC in a party (and beast of burden?) when these rules come into effect, which is undesirable.
Many other attempts have been made to model environmental and exposure effects in later editions of D&D, such as in various Dragon articles, boxed settings (such as the World of Greyhawk), 3rd Edition D&D, etc. (and also by myself, as well). Most of these are moderately complicated and fail the desired criteria of being (a) simple and resolvable by memory, (b) tied into the core D&D mechanic of level-based hit points, and (c) fixed to a sensible game-turn sequence (for example, most rules for weather are applied per-hour, which is then out-of-synch with the standard wilderness turn made per-day, as noted above). In regards to this last consideration, most likely none of those later sources would countenance the "automatic death" clause seen above, but this author considers it to be necessarily game-playable and tastefully old-school.
Thus, for inspiration I've turned to the outdoorsman's elegant "Rule of Three" (quoting Wikipedia):
- Humans cannot survive more than three hours exposed to extremely high or low temperatures, unless individual is using proper gear to cope with heat/cold.
- Humans cannot survive more than 3 days without water.
- Humans cannot survive more than three weeks without food.
Now, some possible criticisms or rule alternatives. First, you might consider delaying any damage assessment until 2 turns have elapsed, based on the "Rule of Three" (thus ensuring that even 1HD creatures retain hit points until 3 turns have gone by). This I would recommend against for the following reasons: (1) It introduces an additional record-keeping requirement (instead of using hit points as the entire count of record). (2) We assume the capacity for full activity during the effect, which is a mitigating factor. (3) Wilderness adventures are generally intended for higher-level characters anyway. (4) Most of us don't play with death at exactly 0 hit points, using some other mechanic for a while thereafter. And (5) a large proportion of even 1HD creatures will survive at least 2 turns even with the existing mechanic (see here).
Secondly, you might consider giving a saving throw against the damage (perhaps half-damage with a save vs. paralysis or dragon breath), which I would personally decline because: (1) This again seems outside the D&D idiom if we look to something like "falling damage" as a model (generally a linear 1d6 per 10 feet fallen, with no save -- noting some alternate suggestions in the past). (2) The advantage would be effectively geometric for higher-level characters (with both greater hit points and saves; something like an O(n^2) effect), allowing them to survive not only significantly longer, but for truly outrageous amounts of time. And (3) you'd have the logistical irritation of needing to roll a save for every PC/NPC/creature in the party over and over again for small amounts of damage, in every turn that the assessment is made.
Therefore, I think the rule above has a lot to commend for itself in terms of elegance, simplicity, and playability.