A major design flaw in many dungeons is the lack of a ventilation
system. This is a shame, since DMs can have the ability to vary
conditions in different parts of the dungeon as a function (or malfunction) of the airflow system. The air can be hot or cold, dry or
damp, pure or putrid, according to the source and amount of ventilation.

All dungeons consisting of more than a few rooms a couple of
feet below ground need ventilation systems if animal life is to survive
(Undead don’t breathe, so they don’t care). Narrow corridors and
closed rooms become death traps if the air doesn’t move—which the
early explorers of the Great Pyramids found out the hard way. To
prevent this, let there be airflow.

VENTILATION SYSTEMS
Designing a ventilation system is elementary—the air needs to go
in and the air needs to go out. This is normally done with air ducts to
every room, though chimneys to the surface from every room are a
possibility.

Ducts
Ducts are typically in the form of tunnels in raw rock a few feet
above the rooms. They will obviously be large enough for the
creatures that made them to crawl through; thus, orc-built ducts will
let almost anyone through, while gnomish ducts will be impassable
for all but gnomes and very non-claustrophobic hobbits.
New duct systems will always have grilles at each outlet, sometimes just rat-proof, but usually quite massive. There will be some
way for creatures to enter the system, since it would require occasional maintenance. Old dungeons may have many grilles missing,
but may instead have traps in the ducts.
Once some grilles are gone, the ventilation system will become a
highway for small carnivores. This could result in animals such as
rats dropping into rooms after fights to eat the dead and wounded.
Ducts also tend to channel sound from room to room.

Chimneys
Straight vertical ventilation shafts are relatively rare, since the
designer would have disliked looters continually dropping things
down them. Each chimney should have at least one sharp horizontal
bend to catch missiles, and a drain at the bend to divert rain, flaming
oil, and the like. If the screens or grilles are missing, bats, thieves and
other undesirables can use chimneys to reach deep levels which they
otherwise couldn’t get to

Pumping Systems
If the dungeon is inside a mountain, the ventilation system can be
designed to use the wind-caused difference in pressure on opposite
sides of the peak to force air through the works. This method requires large and obvious openings that adventurers are sure to find
and exploit.

Dungeons lacking natural airflow will need some kind of pump
ing station using fans, bellows, piston pumps, air elementals, etc.
The mechanical pumps will typically be run by slaves, waterwheels
or volcanic steam.

Pumphouses will always be protected by some means. Slaverun, unprotected pumps might appear on upper levels, but other
more protected types will be on very deep levels, and are used to
supply air to all of the levels above them.

QUALITY CONTROL
Dungeon Masters can make use of decrepit ventilators to give
rooms appropriate atmospheres. Lack of ventilation can in some
cases be used as an attack; just picture the stench of a room full of
zombies after a few weeks. Possible forms of malfunction include:
1. Poisonous air
2. Too hot/cold
3. Room full of smoke, steam or dust
4. Whistling draft
5. Lamp-dousing draft
6. Strange smells
7. “Booming” air pressure (causes deafness)
8. Room in vacuum
9. Room full of water
10. Room full of sewage
A strong stench will tend to forewarn mules and elves, and that
the difference between stinking air and poisonous air (which may be
odorless) is hard for any character to detect. Miners have been
known to bring small birds with them into mines, because birds die
from bad air before people and thus provide them some warning.

WRECKING THE SYSTEM
Eventually players will realize that they can ruin an entire dungeon level by stopping the pumps or filling the intake shaft.
Plan A is to take the pumphouse, stop the pumps, wait for
everything to die, start the pumps again and take all the treasure.
Unfortunately, intelligent dungeon dwellers will notice the lack
of air flow and will try to retake the pumphouse, forming a temporary
alliance among themselves to do so, since their only other choice is
to run for it.

Parties rarely survive Plan A.
Plan B is to find the intake shaft, fill it full of rocks, wait for
everything to die, open up the shaft, and take all the treasure.
Of course, the intelligent monsters will try to unblock the shaft or
drive a new one. Most of the nonintelligent monsters, having
evolved in subterranean settings, would instinctively recognize bad
air— a n d would also attempt to flee to the surface.
Because of the difficulty in winning at vent-wrecking, DM’s need
not worry about it much. Even a party which can defeat any monster
in the dungeon can’t beat all the monsters in the dungeon at once.