In Dungeons & Dragons, Pathfinder, and other fantasy role-playing games, life underground in dungeon and cavern environments is drastically different from that aboveground. Most of these differences are shaped by the unique limitations and opportunities of the underground environment. Several blessings most surface dwellers take for granted, such as sunlight and fresh air, are in short supply beneath the surface. On the other hand, water is usually plentiful underground-even below the driest of deserts, if one is willing to go deep enough-and a variety of rock and mineral treasures await the miner who knows where and how to look.
A particular problem that surface dwellers virtually never face is inadequate air supply. In many underground situations, of course, this is not a problem either: an underground chamber may be so large that even the presence of many creatures does not consume oxygen faster than it can be replaced; or natural or artificially channeled ventilation may ensure that old air is steadily replaced with new. The potential for air supply problems beneath the surface should not be overlooked. If a cave-in occurs, ventilation sources may be cut off; if a chamber is small, it may become so crowded that a meager ventilation system is incapable of replacing oxygen as fast as it is depleted. In any event, air supply is occasionally a factor in underground adventuring.
Assuming no ventilation whatsoever, a human-sized creature at complete rest depletes the oxygen in 10 cubic feet of air within 24 hours. If the creature performs even moderate physical activity, the duration of air supply is reduced to 12 hours. If the creature spends its time in any kind of strenuous exertion, the duration of the air supply is reduced to a mere six hours.
If a character spends some time engaged in heavy or light exertion and rests at other times, simply calculate the oxygen consumption in multiples of the resting rate. Thus, a character who is working strenuously uses oxygen four times faster than a resting character, and a moderately exercising character uses the oxygen up twice as fast as a character at rest. A character who rests for three hours and then works hard for five hours has used up as much oxygen as they would in 23 hours (3 hours + 4 x 5 hours) of rest.
A DM or GM can apply this ratio to all characters and to any creatures, such as mules or dogs, that accompany them. An accurate duration figure can be achieved by figuring the weight of the creature into human equivalents. A dwarf or elf, for example, consumes 2/3 as much oxygen as a human, while a halfling or a dog uses only half as much. A mule or an ogre, on the other hand, consumes oxygen at twice the rate of a human. A character does not instantly die when his air supply has been used up, but serious problems arise. Each turn following the exhaustion of the oxygen, the character must make a Constitution Check. Every time the check fails, each of his ability scores and his hit point total are reduced by one. Each additional Constitution Check is made against the reduced Constitution score.
Thus, a character grows more and more feeble as he gulps the last remnants of the oxygen supply. When all of a character’s ability scores have dropped to 0, he is unconscious. When a character’s hit point total reaches -10, he is dead. Even for a character with a large number of hit points, this fate becomes automatic once the Constitution score reaches 0, since all further Constitution Checks automatically fail. This system is used only for characters in an area with air; it cannot be used for characters who are holding their breath.
If fresh air is introduced to a character before death occurs, they begin to recover immediately. Hit points and ability scores all climb at the rate of three points per turn until the character reaches his original levels in all areas.
Fire is a more voracious consumer of oxygen than even the most heavily working character, and consequently presents a severe threat to characters in situations of limited air supply. Even a flickering torch can. create problems in a small chamber that has little or no ventilation. A small fire (one foot or less in diameter) consumes all of the oxygen in a ten-foot cubic space in two hours (1 2 turns). A flaming torch consumes all of the oxygen in the same space in eight hours.
Of course, when all of the oxygen is consumed, the fire goes out. The oxygen consumption of larger fires is based on the number of small fires they equal. A fire four times larger than a small fire consumes oxygen four times as fast. A flask of burning oil does not burn for very long but uses up a lot of oxygen while it burns. In general, oil in a flask or similar small container burns for three rounds. For purposes of oxygen consumption, however, treat the burning oil as a small fire (as explained above) and treat each round of burning as a full turn. Thus, an oil fire that lasts three rounds uses up as much oxygen as a small wood fire burning for three turns.
The smoke that accompanies fires causes additional problems. While fire consumes the oxygen that characters need to breathe, smoke acts as a poisonous gas that fills enclosed areas and is unavoidably inhaled along with any remaining oxygen. While it is possible to burn extremely dry and flammable materials with little or no smoke, most fires are not this efficient. If any of the fuel for the fire is wet, or if green or living branches, twigs, or leaves are used, smoke is produced. Any kind of burning fabric creates smoke, as does burning oil.
The average amount of smoke produced by a small fire in one turn is enough to create a one-foot-thick smoke layer in a 10 foot x 10 foot square area. The smoke rises to the highest part of a room and collects in a layer against the ceiling. The next turn’s worth of smoke collects immediately below the first layer, and so on, until the breathable air is concentrated in the few feet nearest the floor.
Finally the entire room fills with smoke and even characters lying on the floor are affected. An oil fire creates the same amount of smoke as an average fire, except that in an oil fire the rate is calculated by round instead of by turn. The smoke rate of a largerfire is calculated just as the oxygen consumption, so a fire that is three times as big as a one-foot-diameter fire creates smoke three times as fast. A character forced to breathe smoke must make a Constitution Check each round that the smoke is inhaled. A failed check means that he suffers ld6 hit points of smoke damage.
Smoke damage is temporary. If a character has a chance to breathe fresh air, the smoke damage goes away at the same rate that it was acquired-Le., the character makes a Constitution Check every round of breathing fresh air, with success meaning that they have regained ld6 points. Although temporary, smoke damage is real in the sense that any other damage taken while a character suffers from smoke damage must be considered cumulative with it. Example: A character who has a total of 33 hit points loses 28 of them due to smoke inhalation. If the get struck by an orc for 5 points of damage before they have had a chance to regains any of their smoke damage losses, they are killed.