Players in dungeon adventures are often confronted with situations
involving poison gas, heavy smoke, and various magickal clouds.
DMs who have to do the refereeing on gas attacks find
no help in the existing rules, even for such basic questions as, “How
long until the smoke clears?”

1st, let’s take a look at what we’re dealing with. The three kinds of
noxious vapors normally encountered in dungeons vary considerably
in nature and deadliness, so they should be examined separately.
 

Smoke	Poison gas	Magical clouds	Clearing time	Gas masks
Healing gas damage	-	-	-	Placement of gas traps
Dragon	Poisonous and noxious gases (DSG)	-	Dragon 45	1st Edition AD&D

Smoke
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. A smoke-filled
chamber in a dungeon is inaccessible because no one can function
while coughing and gagging, not because they’ll be dead if they enter.

Poison gas

by Tabletop Witchcraft

Poison gas, on the other hand, should ideally be detectable only in
near-lethal quantities, and should kill very quickly. It should also be
highly volatile so that it will fill a room quickly and then be dissipated
without leaving residue on the room’s surfaces. Three gases that fit
most of these specifications are phosgene, chlorine, and hydrogen
cyanide.

Phosgene is a choking gas which kills in less than 3 hours
through lung damage. It smells like green apples or fresh-cut grass.

Chlorine 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.

Hydrogen cyanide is a colorless gas which smells very faintly of
bitter almonds. It kills by poisoning the central nervous system and by
interfering with oxygen transfer in hemoglobin. Hydrogen cyanide
kills in 15 minutes or less, and is probably the best gas for dungeon
traps.

A typical poison-gas trap is a glass container full of highly pressurized poison gas. 
When the container is cracked it explodes, filling
the room with poison gas almost instantly.

Preparation of poison gases is difficult and dangerous. The
chemical reactions that produce these gases are generally explosive,
and Alchemists find it difficult to make equipment sturdy enough to
keep from killing all involved. 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 GP each

Chlorine is prepared by dripping hydrochloric acid (from animal
stomachs) onto pyrolusite (a mineral). Alternatively, it can be obtained
from Green Dragon’s breath.

Phosgene is prepared by mixing chlorine and carbon monoxide in
the presence of activated charcoal at 200 degrees C.

Hydrogen cyanide is prepared by passing a mixture of ammonia
(distilled from the horns of oxen), methane (from dung), and oxygen
(made by heating mercuric oxide) through a slowing platinum gauze.

There are many other poison gases, as well as semi-poisonous
gases, which do not kill instantly, but are good for “smoking out”
enemies. Semi-poisonous gases include ammonia, burnt sulfur, and
many others.

Magickal clouds
Magical clouds come in many different forms. They obviously don’t
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 — breezes blow
them around without blowing them apart. Magical clouds expand
until they fill the volume listed in the spell description, and then stop.
They don’t keep growing, they don’t diffuse into the surrounding air,
and they resist being pushed out of shape. A large cloud will resist
being sucked down a small ventilation shaft, so room ventilation tends
to have little or no effect on magical clouds.

Clearing time
Now that we know what we’re dealing with, let’s look at how to
handle noxious vapors in the dungeon.

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.

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.

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 1/800 the concentration preferred 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 concentration.
Using the 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 to room volume is about 1:20.83.

This doesn’t mean that all of the air in the room is going to be replaced 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 decaying exponential curve.

    For those who care, the function is
            C(t) = C(0)exp(-tVi/Vr)
                Where
            C(t) = concentration of poison gas at time t,
            C(0) = initial concentration of poison gas,
            Vi is the rate of ventilation in cubic feet per minute,
            Vr is the room volume in cubic feet.

Those who don’t care don’t have to know how to handle the
algebra to apply the results.

Using the standard ventilation, the time for the smoke in a room to
clear to 10% of its original value is about 50 minutes. For
poison gas to set down to 0.125 percent takes 2 hours and 20 minutes.

If you figure that the ventilation is better than average in a particular
room (as it would be in places like Alchemists’ workshops), then you
guess at how much better it is and divide the Time by that amount. For
example, if you had a smoke-filled room with ventilation 5 times
better than normal, the time to clear would be 50 minutes divided by 5,
or 10 minutes. For inferior ventilation you increase the time to
clear; if the smoke-filled room had only 1/2 normal ventilation,
the time to clear would be twice as long, or 100 minutes.

If there’s no ventilation at all, poison gas will NOT hang around forever. Poison gases are highly reactive — that’s what makes them so
deadly — 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’s not a very fast process, so for gaming purposes let’s 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 corroded, scrolls will be bleached clean, locks will be rusted shut, etc.

A sealed room filled with smoke would remain foul forever, since
there is a lack of oxygen in addition to the presence of poisons. Unsealing 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.

These figures assume that the initial concentrations of poison gas or
smoke are always the same. They aren’t, of course, but we can rationalize an excuse for this assumption, as follows:

Gas masks
Sometimes an enterprising PC decides that he needs a
gas mask.

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
AD&D universe the gas mask hasn’t been invented yet.

If you allow a PC 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 finding 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 GP, with a
50% chance that the gas mask design doesn’t really work.

Healing gas damage
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.

Death does not occur instantly. The character is immediately unconscious, but doesn’t actually die for 5 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 and Neutralize
Poison spells are effective on poison-gas victims, but general wound-curing spells are not.

Placement of gas traps
Poison gas is a very nasty kind of trap, and DMs
should use it sparingly at low levels. Even for advanced players, 
an improperly run poison-gas trap can fall into the “instant death, no saving
throw” category, which causes people to switch to someone else’s
campaign in a hurry. Smoke is more mundane, and any Time the party
does something stupid with flaming oil or fireballs the DM shouldn’t
hesitate to let everything smoke and burn that can do so. A dungeon
should be designed to be survivable, but during the expedition itself
the DM should never let the players get away with anything, no matter
how much they grovel.

Table 1
Poisonous and semi-poisonous gases

Poison Gases	Type	Rate of Action	Odor
Phosgene	Choking gas	Instant to 3 hrs	New-mown hay
Chlorine	Choking gas	Instant to 3 hrs	Bleach
Distilled Mustard*	Blister gas	Delayed 4 to 6 hrs	Garlic
Nitrogen Mustard*	Blister gas	Delayed 12 or more hrs	Fishy or musty
Tabun, sarin, soman*	Nerve gases	0-15 minutes	None when pure
Hydrogen cyanide	Blood gas	0-15 minutes	Bitter almonds
Cyanogen chloride*	Blood gas	Immediate	None
Arsine*	Blood gas	2 hrs to 11 days	None
Semi-Poisonous Gases	Type	Rate of Action	Odor
Ammonia	Tear gas	1 minute	Ammonia
Burnt sulfur	Choking gas	1 minute	Sulfur
Adamsite*	Vomiting gas	1 minute	None
Chloracetophenone*	Tear gas	1 minute	Apple blossoms

* — indicates a gas beyond the ability of Alchemists to produce.

Semi-poisonous gases cause damage at the rate of 1-4 hit points
per melee round of exposure. Exposure to smoke causes 1 point of
damage per round.

Choking gases cause death from lung damage.

Blister gases destroy tissue; especially in moist areas, such as the
lungs and mucous membranes.

Blood gases are systemic poisons, directly affecting heart or nerve
action.

Nerve gases inhibit an enzyme, which allows accumulation of the
toxin acetylcholine.

Vomiting gases and tear gases induce vomiting and tears, as you
would expect.

Information on poison gases was obtained from the Encyclopaedia Brittannica, 1969. Vol. V, pp. 382-387.