Rule of Three: Environmental Damage

Classic D&D (OD&D/AD&D) somewhat oddly lacked any core rules for exposure to environmental factors such air, heat, and food. Here's my offering for the shortest possible rules to handle those situations; justification and analysis comes afterward. The text between the horizontal rules is designated as Open Game Content.

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Rules for Exposure
Characters lacking certain physical necessities accrue 1d6 damage per time unit, as outlined below:

• No Air: 1d6 per minute.
• No Heat: 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. When the exposure condition ends, hit points are regained at the same rate they were reduced (1d6 per time unit).

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Justification for the Rule

In brief, the rule is inspired by the common outdoorsman's "Rule of Three" which dictates how long a human can typically survive: 

1. 3 minutes without air.
2. 3 hours without shelter (heat/cold)
3. 3 days without water
4. 3 weeks without food.

A few example blogs where this is expressed: one, two, three. So, it occurred to me to simply assess damage at the same time-units as indicated above, which generally results in hit points being zeroed out for any introductory character (levels 1-3) in about 3 turns, as indicated. Note that the "no air" assessment during combat may differ significantly based on whether you play with 1 round = 1 minute (i.e., once per round), 1 round = 10 seconds (i.e., once every 6 rounds), or something else.

Now, 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-sync with the standard wilderness turn made per-day, as noted above). I

Back to the more elegant "Rule of Three" alternative: some possible criticisms arise. 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 record of account). (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.

In some sense, the rule is best calibrated for PCs of around 3rd level (for obvious reasons). That said, despite the harshness for 1st-level characters, I think the basic rule above has a lot to commend for itself in terms of elegance, simplicity, and playability.