Poisonous and Noxious Gases in Dungeons & Dragons and other role-playing games can originate from a number of underground sources. Volcanic activity can create deadly emissions of Sulphuric gas that are every bit as poisonous as the bite of a venomous creature, while garbage or sewage can generate noxious gases such as methane whose smells are so overpowering that characters have a difficult time breathing them.
The occurrence of such gases is primarily a campaign function to be handled by a DM or GM at the appropriate time. As in most situations involving poison, poisonous gases generally require characters to make saving throws vs. poison. Noxious gases should force characters to make Constitution Checks every round; failure results in a general lowering of ability scores by three, with a similar penalty applied to attack rolls. The modern example of tear gas falls into this category. Characters can be rendered quite helpless by the effects of noxious gases (when all ability scores are reduced to 0), but they are not killed because there is no loss of hit points. Ability scores are raised at the rate of three per turn when the character has a chance to breathe fresh air again. Modifiers to the saving throw or Constitution Check can be applied as the DM or GM sees fit, to account for exceptionally lethal or mild poisons or to simulate degrees of noxiousness.
Certain types of gases, such as natural gas, are either odorless or possess such a mild odor that the characters may not be aware that the gas is present until they start making checks or saving throws. Other types of gases, such as Sulphur, carry such a strong odor that the characters might have a chance to hold their breath before the full force of the gas can take effect.
Natural gas, often encountered in regions where coal and oil are common, has the additional hazard of being explosive. If characters encounter natural gas and are carrying a torch, candle, lantern or any other source of open flame, the natural gas has a base 20% chance per round of exploding. Such an explosion causes 1 d6 points of damage to all characters and creatures in the area with the gas. Characters entering a region containing natural gas in such quantities might be allowed to make Wisdom Checks if they are moving slowly and investigating their surroundings carefully.
More heavily concentrated natural gas, or other types of explosive gases, are possible. The DM or GM can modify the gas’s chance of exploding and damage inflicted upward as follows: each 10% increase in the chance of explosion adds another d6 to the damage inflicted to the characters. Higher concentrations should also increase the characters’ Wisdom scores by 1 per 10% potency increase, for purposes of this Wisdom Check only.
Note that if a gas is completely odor-free, no Wisdom Check should be allowed, since the check represents the character’s awareness of the gas’s odor and an odorless gas gives no opportunity for a check. Any gas, whether poisonous, noxious, or harmless, can cause problems to characters by replacing the oxygen in a given area. In this case, the effects should be treated as if part or all of the ‘Oxygen in a location has been used up, as explained earlier.
Certain creatures, most notably small birds, are more sensitive to poisonous gases than humans and other character races are. If, perchance, a party has a small bird with them and becomes exposed to a gradually increasing amount of poisonous gas (not including noxious gases), the DM or GM should make a secret saving throw vs. poison for the bird one turn before the characters must save. The bird saves as a Oth-level human, and if the saving throw fails, the bird dies. Note that this procedure is of little help if gas is suddenly introduced to an area in high concentration. The party must encounter a gradually increasing amount of gas in order for a bird to provide any early warning.
Ventilation of an underground area depends on several factors. Chief among these is fresh air flowing into and through the area. Any underground chamber with a noticeable airflow is considered well-ventilated. Smoke does not accumulate in such an area, nor is the oxygen exhausted by characters breathing or by fire. The duration of all gas-based spells is halved in such an area. Poisonous or noxious gases remain in a ventilated area for two turns unless more of the gas is introduced.
More subtle air movement can be detected by the flickering of a torch or candle. Even when characters cannot discern this movement through their own sense of touch, fresh air for breathing is not a problem in these areas. Gas-based spells linger for their normal durations, and poisonous or noxious gases remain effective for one hour. Smoke from small fires or torches does not accumulate in moderately ventilated areas, while smoke from large fires or from burning oil only accumulates at 1/2 the rate listed for enclosed areas. Some areas appear to contain no ventilation, but are actually connected either to ventilated areas or to the earth’s surface. No matter how long or winding the connecting passages may be, these areas collect smoke from small fires at 1/2 the rate previously listed. Large or oil fires fill the area with smoke at the usual rate.
Characters do not use up all of the air through breathing; even this minimal amount of ventilation allows enough air movement for respiration. Poisonous or noxious gases remain in such an area for 2-5 hours (ld4+1). The duration of all gas-based spells is normal. An area that contains no noticeable ventilation and has no place for smoke to flow out horizontally or upward captures smoke at the normal rate (even from small fires). Poisonous or noxious gases, unless specifically noted as being heavier than air, remain in such an area for a full 24 hours.
Areas that are completely sealed off from any outside passages, either through cave-ins, constructed barriers such as mortared stone walls, or magical barriers such as walls of force, are subject to all of the effects listed, at their normal rates of accumulation and effect.
The air supply is finite, and once used up, cannot be replaced except through the introduction of fresh air. Smoke that collects in such a room lingers for ld6 hours after the fire that created it goes out. Poisonous or noxious gases remain in the area until vented out by the admission of fresh air. Again, the duration of all gas-based spells is normal.
The sense of smell, while relatively insignificant when compared to those of sight and sound, occasionally provides a character with advance warning of some nearby hazard or obstacle. This sense is only useful to a character when no other overpowering scent is in close proximity to him or permeating the area. The basic procedure for a character to detect an odor is a Wisdom Check made when the scent is first encountered. Another check may be made if the scent grows very strong, or a change in the wind carries it to the character with increased force.
Of course, this Wisdom Check should be made by the DM or GM so that the players are not alerted that they should be on the lookout for something. The reason that the check is not repeated more frequently is that if an odor is not noticed when it is first encountered, it will probably not be noticed thereafter unless something occurs to draw the characters’ attention to it. If the party has any dogs accompanying them, the DM or GM may also check for the dogs if they would be alerted by the scent.
The scent of a monster would almost certainly alert them, for example, while the odor of a campfire probably would not. Dogs are all assumed to have a Wisdom of 16, for purposes of this check only. If a dog smells something, the DM or GM should describe the dog’s reaction in appropriate terms: a growl of warning, perhaps, or a cringe of fear. If a dog has been trained to react in a certain way, then this reaction should be used. Sophisticated responses require considerable training, however, and should only be used if the dog has been trained.
The Wisdom Check is only made using the character’s full Wisdom score if the odor is strong enough to be easily noticed. The strength of an odor is obviously going to require a DM or GM judgment call, but general guidelines (for distance, type of odor, etc.). If a dog or animal like is making the check, consider the animal and modify accordingly.