Monday, September 13, 2010

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.

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).

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.


  1. It does look quite workable and would allow for interesting things to happen just like in real life. Just think of the reasons a player could come up with for continually rolling 1s on a dehydration roll. Only one thing that I would add would be 1d6 for resting over night in mild exposure because at 0 Celsius you won't die quite that fast but overnight outside with only the clothes on your back and you just might.

  2. Looks good to me. Except for shelter:
    " * No Shelter: 1d6 per hour. *

    * Assumes exposure to extreme temperatures, either below -20°F (-30°C) or above +90°F (+30°C)."

    Folks can die of hypothermia when temps are in the 40's (F), certainly so if they get wet and
    I've spent plenty of days recreating outside all day when temps were over 90 and no one died.
    Lack of shelter really is relative and can be offset by good sense and gear (that isn't really shelter).

    I posted a similar system that is more fiddly for Mutant Future based on the very handy rules of three.

  3. I've always heard the rule of three described as:

    3 minutes without air
    3 days without water
    3 weeks without food

    Granted, that's before real harm, not necessarily DEATH. Likewise, that's also not covering something like a person undergoing a careful fasting regimen.

    Re: Air. 1d6 damage per minute is a bit low. Especially if that's supposed to be AFTER they "Run out of air." (IMO)

  4. I think you have to define what counts as shelter. It seems like you mean a space that blocks the elements, and perhaps can be warmed, but maybe it's just shade. Unclear. 90F as extreme heat is too low; maybe it has to 110 or 120F. I work with Maya farmers in southern Belize that clear acres of bush by hand with machetes in (humid) 90F, and they aren't dead after, say, 3 hours. And for the unacclimated it does kick your ass, but you adapt to it in a few days, assuming you have adequate water.

    Usually folks rest during the hottest parts of the day in the tropics. Perhaps a traveling party should be forced to do this or else incur some kind of penalty, maybe 1d6 per siesta worked through, but 1d6 not per hour.

  5. Just a few nitpicks

    firstly, I think the recovery time is too quick. All of these things are very hard on the human body, probably at about the rate you describe (in 3/4 I would assess damage with d10s instead), but recovery shouldn't be as fast as infliction.
    I would have the recovery time be twice what it took to sustain the damage, but take away the -2 penalties after the first unit of time so they're no longer dulled by the experience, but still not okey-dokey.

    Also, no air seems pretty damn weak in this example. It's quickly fatal to level 1 characters, but IRL even experienced swimmers can be fatally affected in ten minutes. I don't want to make it extra bloodthirsty, but I do want it to take you out of commission faster, so can we say that at 1/2 or 1/3 your maximum HP that no air renders you unconscious? This way a 12th or 15th level fighter gets 4-6 minutes of activity before going beddy-bye, but can still be rescued, while a first level mage with no constitution bonus will be taken out of commission in short order.

  6. I also think the "no shelter" should be defined to include gear like clothing. If you have lots of furs and such you should be okay traveling in arctic weather - perhaps change the damage interval from "hour" to "day" in that case.

    But the air thing makes sense to me. As far as I'm concerned, few people in the real world would be the equivalent to a 5th or 6th level character in D&D, and an experienced swimmer could certainly still be 1st level. Look at that as a problem with how D&D models things. Point is, D&D does not model real life, it models the literary source material. Watch an action flick and see how long the protagnist gets to hold his breath. A PC with d6 HP surviving in airless space for 1 minute per HD on average is not a problem. If your game uses d4 to d12 for HP, you should probably be rolling d8 or d10 for this environmental damage. And if your games have characters of 20th to 30th level as a regular standard, you should be used to reconciling rules that break down at such extreme levels.

  7. Great comments guys, really appreciate the feedback. I think I should update this at least a little bit.

    On Air (bighara, GragSmash) -- First, it's not "after air runs", it's assessed immediately once a no-air condition begins. In the past I would agree and try to make this more real-world accurate, but now I agree with 1d30. If a high-level character can fight off a dozen men or survive a fall from 120 feet, I think it's in-spirit to allow them 12 minutes of time without air. Much like falling, there are some rare cases of people surviving 40 or 70 minutes underwater. Link.)

    On High Temperatures (JDJarvis, Spawn of Endra) -- Here I was looking at a standard 4-part chart of Heat Index risks, and using the 90-105 "Extreme Caution" class as the base. I think I'm convinced that should be notched up to the 105-130 "Danger" category. (Link.)

    On Low Temperatures (JDJarvis, 1d30) -- Here I'm looking at typical Wind Chill charts, and sort of basing it where chance of frostbite begins. This is generally tougher to gauge because it does include clothing moreso than heat. My assumption here is that people are clothed in dry, typical Medieval Europe-style cold weather clothing (e.g., Viking, Mongols), and thus some part of the body is exposed to possible frostbite. Perhaps I should notch this up a bit to -15F (-25C). (Link.)

    On Recovery (GragSmash) -- I think I'd need a more specific citation. The only thing I can find on any of these issues is the Minnesota Starvation Experiment where the recovery took less time than the half-starvation period. (Link.)

  8. @Delta

    Point conceded on recovery -- pretty counter-intuitive, I'll admit to being surprised.

    You don't agree on having the risk of passing out before running out of hit points? seems pretty reasonable, but not very heroic I guess.

    IMHO, one of the things 3.x gets right is the idea of npc classes, but I do think their execution could be simplified for old-school sensibilities to save on prep time.

    An olympic-class diver is not a first level character -- he had to spend a lot of time training to get where he is -- now he can't claim all the abilities of a fighter of his level, but if you put athletes, and laborers (like blacksmiths, carters, etc) into some NPC class, you could give them moderate attack bonus growth and similar hitpoints to a cleric, but not have to prepare more than any other character.
    Also allows a GM to have npc tradesfolk with some salt to them without having to say "well he used to be an adventurer"

    more white-collar workers (scribes, bankers) might be in a different NPC class with similarly anemic hitpoint and combat growth to a wizard, and assess an x-in-6 chance of them succeeding in whatever work is suitable to their profession.

    I may write something a little more detailed up for my much-neglected blog, but this is hopefully an understandable gist of my objection to the notion that any non-adventurer would be level 0. It's not even necessary to say every tradesman is level x, just that certain ones who excel at their craft have levels.

  9. I like it, I'm stealing it. But one question: why can't it be healed like other stuff? Healing is healing, unless it is some supernatural wound.

  10. @Justin: Because (a) It's not a "wound/hit", and (b) If allowed, it would let you go indefinitely without food/water by application of "cure light wounds", and (c) It would render 4th/5th level create water/food magic basically irrelevant.

  11. Re: wounds and cures, but a loss of hit points in combat is not necessarily a wound/hit either, strictly speaking.

    I'd allow cures to fix people up. It's the magical version of CPR!

  12. @crom: "Re: wounds and cures, but a loss of hit points in combat is not necessarily a wound/hit either, strictly speaking."

    One of those points I don't agree with, speaking of classical D&D. (Possibly excepting phantasmal forces.)

  13. There are some exposure rules in my B/X Companion, though I treat lack of air to be the same as drowning/asphyxiation.

  14. @JB: Right, that's the type of thing I'm responding to, noting (RC p. 89-90): (1) air rule is fairly complicated, (2) heat/cold rule nonexistent, (3) water rule nonexistent, (4) food rule doesn't specify numerical penalties (a list of "could'ves").

  15. cr0m: "I'd allow cures to fix people up. It's the magical version of CPR!"

    Thinking on this, I think CLW is definitely more like "first aid" than CPR. Consider: (1) Wikipedia "first aid" and the first thing you see mentioned are religious Knights Hospitallers (alluded to in AD&D cleric description), (2) analogous AD&D barbarian ability uses that name, (3) TSR Star Frontiers skill analogues to "cure wounds" sequence are called "first aid/ minor surgery/ major surgery".

  16. This is really, really elegant. All of the rules should have been this good.

  17. I suppose if we wanted a mitigating factor for low-HD figures (that doesn't geometrically help high-level figures), we could say: damage 1d6, to a maximum of 1/3 the figure's full hit points. Or something.