{"id":97,"date":"2026-09-06T17:54:03","date_gmt":"2026-09-06T17:54:03","guid":{"rendered":"https:\/\/through-dungeons-deep.com\/wordpress\/?p=97"},"modified":"2026-09-06T18:57:31","modified_gmt":"2026-09-06T18:57:31","slug":"gas-em-up-and-smoke-em-out","status":"publish","type":"post","link":"https:\/\/through-dungeons-deep.com\/wordpress\/gas-em-up-and-smoke-em-out\/","title":{"rendered":"Gas \u2018em up and smoke \u2018em out"},"content":{"rendered":"

\"Dragonby Robert Plamondon<\/h4>\n

First published in Dragon Magazine #45, January, 1981.<\/em><\/p>\n

Note:<\/strong> This is the first of two related articles, both published in Dragon #45.<\/em> The other is Dungeon Ventilation Clears the Air.<\/p>\n

Players in dungeon adventures are often confronted with situations involving poison gas, heavy smoke, and various magical clouds. Dungeon Masters who have to do the refereeing on gas attacks find no help in the existing rules, even for such basic questions as, \u201cHow long until the smoke clears?\u201d<\/p>\n

Three Different Types<\/h2>\n

First, let\u2019s take a look at what we\u2019re dealing with. The three kinds of noxious vapors normally encountered in dungeons vary considerably in nature and deadliness, so they should be examined separately.<\/p>\n

Smoke<\/h3>\n

Smoke is the least lethal of the gases found in dungeons. Although smoke contains carbon monoxide and other unpleasant substances, it rarely kills adventurers because the unpleasant effects of smoke force everyone to flee long before the lethal effects arise (unless they’re trapped, of course). A smoke-filled chamber in a dungeon is inaccessible because no one can function while coughing and gagging, not because they\u2019ll be dead if they enter.<\/p>\n

\"\"Poison gas<\/h3>\n

Poison gas, on the other hand, should ideally be detectable only in near-lethal quantities, and should kill very quickly. (That is, if you\u2019re the person deploying it. If you\u2019re on the receiving end, you may hold a different opinion.) It should also be highly volatile so that it will fill a room quickly and then be dissipated without leaving residue on the room\u2019s surfaces. Three gases that fit most of these specifications are phosgene, chlorine, and hydrogen cyanide.<\/p>\n

    \n
  • Phosgene<\/em> is a choking gas which kills in less than three hours through lung damage. It smells like green apples or fresh-cut grass.<\/li>\n
  • Chlorine<\/em> is a greenish-yellow gas which kills in the same manner as phosgene. It has a very strong odor (like bleach), but is suitable for traps where the gas is released from a sealed container. Chlorine kills in just a few minutes.<\/li>\n
  • Hydrogen cyanide<\/em> is a colorless gas which smells faintly of bitter almonds. It kills by poisoning the central nervous system and by interfering with oxygen transfer in hemoglobin. Hydrogen cyanide kills in fifteen minutes or less, and is probably the best gas for dungeon traps.<\/li>\n<\/ul>\n

    A typical poison-gas trap is a glass container full of a pres\u00adsurized poison gas. When the container is cracked it shatters, filling the room with poison gas almost instantly.<\/p>\n

    Preparation of poison gases is difficult and dangerous, but that’s what Alchemists are for. The chemical reactions that produce these gases are generally explosive, and Alchemists find it difficult to make equipment sturdy enough to prevent killing all involved. In a medieval setting, this tends to keep poison gas rare and expensive. Flasks of poison gas recovered from dungeons are salable to Alchemists for 500 to 1,500 gold pieces each<\/p>\n

      \n
    • Chlorine is prepared by dripping hydrochloric acid (from animal stomachs) onto pyrolusite (a mineral). Alternatively, it can be obtained from Green Dragon\u2019s breath. Have fun!<\/li>\n
    • Phosgene is prepared by mixing chlorine and carbon monoxide in the presence of activated charcoal at 200 degrees C.<\/li>\n
    • Hydrogen cyanide is prepared by passing a mixture of ammonia (distilled from, among other things, the horns of oxen), methane (from dung), and oxygen (made by heating mercuric oxide) through a slowing platinum gauze.<\/li>\n<\/ul>\n

      Nerve gas, mustard gas, and other dubious advances of the 20th<\/sup>\u00a0century are included in the table but are not discussed here: too advanced.<\/p>\n

      Semi-Poisonous\u00a0Gas<\/h4>\n

      Semi-poisonous gases do not kill instantly, or perhaps at all, but are good for \u201csmoking out\u201d enemies. I don’t cover them here. Semi-poisonous gases include ammonia, burnt sulfur, and many others.<\/p>\n

      Magical clouds<\/h3>\n

      Magical clouds come in many different forms. They don\u2019t last any longer than the duration of the spell, but what happens as you try to dilute a magical cloud is not entirely clear. In my campaign I treat magical clouds as objects that resist dispersion \u2014 breezes blow them around without blowing them apart. Magical clouds expand until they fill the volume listed in the spell description, then stop. They don\u2019t keep growing, they don\u2019t diffuse into the surrounding air, and they weakly resist being pushed out of shape. Thus, a large cloud has some resistance to being sucked down a small ventilation shaft or even passing through a grille, so room ventilation tends to have little or no effect on it.<\/p>\n

      Clearing time<\/h2>\n

      Let\u2019s look at how to handle noxious vapors in the dungeon.<\/p>\n

      The ventilation system (provided there is one) will, in time, flush away poison gas. There is a simple method for finding how much is left at any time after the gas is introduced.<\/p>\n

      In the case of a smoke-filled room it seems reasonable to assume that when 90% of the smoke is gone, the room will have a bearable atmosphere.<\/p>\n

      For poison gas, however, even tiny concentrations can cause permanent damage. For example, chlorine is safe for only short exposures in doses as low as one eight-hundredth the concentration pre\u00adferred in warfare. This means that dungeon explorers will have to wait until the poison gas in a room is less than 0.125% of its original con\u00adcentration.<\/p>\n

      Ventilation and Clearing Time<\/h3>\n

      Using the industrial rule-of-thumb design specs of 500 cubic feet per person of room volume and 24 cubic feet per minute per person of ventilating air, and applying a little algebra, we find that the ratio of incoming air volume per minute to room volume is about 1:21 in a space designed for human occupancy, and with airflow that\u2019s working as designed. Spaces not designed for continuous occupancy may have much less. Is your dungeon’s ventilation system well-designed and well-maintained enough to work at twentieth century specifications? Probably not.<\/p>\n

      This doesn\u2019t mean that all of the air in the room is going to be re\u00adplaced in 21 minutes, because the new air mixes with the old air and this mixture is what leaves the room. The solution is actually a decay\u00ading exponential curve. (This is called a mixing problem,<\/em> by the way.)<\/p>\n

      For those who care, the function is:<\/p>\n

      \"Gas<\/p>\n

      Where<\/p>\n

        \n
      • C(t)<\/em>\u00a0= concentration of poison gas at time\u00a0t,<\/em><\/li>\n
      • C(0)<\/em>\u00a0= initial concentration of poison gas,<\/li>\n
      • Vi<\/em>\u00a0is the rate of ventilation in cubic feet per minute,<\/li>\n
      • Vr\u00a0<\/em>is the room volume in cubic feet.<\/li>\n<\/ul>\n

        Those who don\u2019t care don\u2019t have to know the algebra to apply the results.<\/p>\n

        Using the standard ventilation, the time for the smoke in a room to clear to ten percent of its original value is about fifty minutes. For poison gas to set down to 0.125 percent takes two hours and twenty minutes.<\/p>\n

        If you figure that the ventilation is better than average in a particular room (as it would be in places like Alchemists\u2019 workshops), then you guess how much better it is and divide the time by that amount. For example, if you had a smoke-filled room with ventilation five times better than normal, the time to clear would be fifty minutes divided by five, or ten minutes. For inferior ventilation you increase the time to clear; if the smoke-filled room had only one-half normal ventilation, the time to clear would be twice as long, or 100 minutes.<\/p>\n

        What if There’s No Ventilation?<\/h3>\n

        If there\u2019s no ventilation at all, poison gas won\u2019t hang around for\u00adever. Poison gases are highly reactive \u2014 that\u2019s what makes them so deadly \u2014 and will form relatively harmless compounds eventually. Chlorine, for example, will bleach everything in the room until all of it is combined with something. Although there is no way of telling exactly how long this would take in a typical slimy dungeon chamber, it\u2019s not a very fast process, so for gaming purposes let\u2019s say that it takes a month for poison gas to dissipate in a sealed room. Since all of the poison gases are corrosive, non-magical metals will be heavily cor\u00adroded, scrolls will be bleached clean, locks will be rusted shut, etc.<\/p>\n

        A sealed room filled with smoke would remain foul forever, since there is a lack of oxygen in addition to the presence of poisons. Un\u00adsealing the room and waiting a few hours to let some fresh air in should work for smaller rooms. For huge chambers the waiting time could be weeks or months, due to the poor air circulation and large room volume.<\/p>\n

        These figures assume that the initial concentrations of poison gas or smoke are always the same. They aren\u2019t, of course, but we can ration\u00adalize an excuse for this assumption, as follows:<\/p>\n

        Poison gas is very expensive. Costs run between 1,000 and 6,000 gold pieces per trap, depending on local variations in expenses and the amount of gas made. Prices are high because the people who make\u00a0it\u00a0are putting themselves in considerable peril, and get paid ac\u00adcordingly. No one is going to waste expensive poison gas by using ten times too much in a trap; he\u2019s also not going to use much too little and render his trap useless. Thus, we can expect that the trap (or what\u00adever) was loaded with just enough poison gas to fill the chamber it\u2019s in. If this is the case, the initial concentration of poison gas will be uniform from trap to trap.<\/p>\n

        The figures for smoke assume that the fire used up all of the room\u2019s oxygen. In other words, the smoke is just as thick as it can be. For lesser fires the smoke would clear faster, just as if it were in a better- ventilated room.<\/p>\n

        Magical Clouds vs. Ventilation<\/h3>\n

        Which still leaves us with magical clouds to deal with. Since the magical clouds are hard to blow apart, they would tend to remain in the room for the duration of the spell. If the room is larger than the cloud, however, the cloud will drift over to the side with the ventilation exhaust (assuming no spell-caster is present to tell it where else to go). This might allow the adventurers to enter the room before the spell quits.<\/p>\n

        Gas Masks<\/h2>\n

        Sometimes an enterprising player character decides that he needs a gas mask.<\/p>\n

        Gas masks are fairly simple devices mechanically, the active part being made of activated charcoal and soda lime. They would be easy for an alchemist and a leather worker to put together, but in the\u00a0AD&D<\/em> universe the gas mask doesn’t seem to have been invented yet.<\/p>\n

        If you allow a player character to think of the idea of a gas mask, he will be able to attempt to find an Alchemist to take on the job of find\u00ading out how to make one. This will take time and a lot of money; I would suggest from 2-8 months and 2,000-7,000 gold pieces, with a 50% chance that the gas mask design doesn\u2019t really work.<\/p>\n

        Healing Gas Damage<\/h2>\n

        Poison gas is easy to referee. The player is entitled to a saving throw; if he makes his saving throw he manages to hold his breath and leave the gas, taking no damage. If he fails, he breathes the gas and dies.<\/p>\n

        Death does not occur instantly. The character is immediately un\u00adconscious, but doesn\u2019t actually die for five rounds. This allows others to go back into the cloud (and roll another saving throw at +4 to see if they die this time) and rescue the victim. Slow Poison<\/em>\u00a0and\u00a0Neutralize Poison<\/em> spells are effective on poison-gas victims, but general wound \u00adcuring spells are not.<\/p>\n

        Placement of Gas Traps<\/h2>\n

        Poison gas is a very nasty kind of trap. I would not use it at low levels or with inexperienced players. Even for advanced players, an im\u00adproperly run poison-gas trap can fall into the \u201cinstant death, no saving throw\u201d category, which causes people to switch to someone else\u2019s campaign in a hurry. Smoke is more mundane, and any time the party does something stupid with flaming oil or fireballs the DM shouldn\u2019t hesitate to let everything smoke and burn that can do so. A dungeon should be designed to be reasonably survivable, but if the players insist on barbequing their own characters, what’s a DM to do?<\/p>\n

        Table of Gases and Their Effects<\/h2>\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n
        Poison Gases<\/strong><\/td>\nType<\/strong><\/td>\nSpeed<\/strong><\/td>\nOdor<\/strong><\/td>\n<\/tr>\n
        Phosgene<\/td>\nChoking gas<\/td>\nInstant to 3 hrs<\/td>\nNew-mown hay<\/td>\n<\/tr>\n
        Chlorine<\/td>\nChoking gas<\/td>\nInstant to 3 hrs<\/td>\nBleach<\/td>\n<\/tr>\n
        Distilled Mustard*<\/td>\nBlister gas<\/td>\n\n

        4 to 6 hrs<\/p>\n<\/td>\n

        		Garlic<\/td>\n<\/tr>\n
        Nitrogen Mustard*<\/td>\nBlister gas<\/td>\n12 or more hrs<\/td>\nFishy or musty<\/td>\n<\/tr>\n
        Tabun, Sarin, Saman*<\/td>\nNerve gases<\/td>\n0-15 min<\/td>\nNone when pure<\/td>\n<\/tr>\n
        Hydrogen cyanide<\/td>\nBlood gas<\/td>\n0-15 min<\/td>\nBitter almonds<\/td>\n<\/tr>\n
        Cyanogen chloride*<\/td>\nBlood gas<\/td>\nImmediate<\/td>\nNone<\/td>\n<\/tr>\n
        Arsine*<\/td>\nBlood gas<\/td>\n2 hrs to 11 days<\/td>\nNone<\/td>\n<\/tr>\n
        Semi-Poisonous Gases<\/strong><\/td>\n<\/tr>\n
        Ammonia<\/td>\nTear gas<\/td>\n1 minute<\/td>\nAmmonia<\/td>\n<\/tr>\n
        Burnt sulfur<\/td>\nChoking gas<\/td>\n1 minute<\/td>\nSulfur<\/td>\n<\/tr>\n
        Adamsite*<\/td>\nVomiting gas<\/td>\n1 minute<\/td>\nNone<\/td>\n<\/tr>\n
        Chloracetophenone*<\/td>\nTear gas<\/td>\n1 minute<\/td>\nApple blossoms<\/td>\n<\/tr>\n
        * Indicates a gas beyond the ability of Alchemists to produce.<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n