Atmospheric Pressure

Regardless of its composition, an
atmosphere may be difficult or impos-
sible to breathe if its pressure is wrong.
We measure air pressure in “atmos-
pheres” (atm.); 1 atm. is air pressure at
sea level on Earth.
Trace (up to 0.01 atm.): Treat an
atmosphere this thin as vacuum (see
Vacuum, p. 437).
Very Thin (up to 0.5 atm.): The air
is too thin to breathe. Earth’s atmos-
phere becomes “very thin” above
20,000’. If you lack protection (e.g., the
Doesn’t Breathe advantage, or a respi-
rator and oxygen tanks), you suffocate
– see Suffocation (p. 436). Vision rolls
are at -2 without eye protection.
Thin (0.51-0.8 atm.): Earth’s atmos-
phere is “thin” between 6,000’ and
20,000’. Thin air is breathable if oxy-
gen is present in Earthlike percent-
ages, but it is hard on unprotected
individuals. Increase all fatigue costs
for exertion by 1 FP. Vision rolls are at
-1 without eye protection. Finally, any-
one who breathes thin air for an hour
or more must check for “altitude sick-
ness.” Make a daily HT roll at +4.
Critical success means acclimatiza-
tion – do not roll again. Success
means no effect today. Failure means
headaches, nausea, etc., giving -2 to
DX and IQ. Critical failure means the
victim falls into a coma after 1d hours;
see Mortal Conditions (above). Roll
against Physician skill once per day to
revive the victim before he dies.
Dense (1.21-1.5 atm.): The air is
breathable, with some discomfort: -1
to all HT rolls, unless you have a pres-
sure suit. If the air contains more than
50% oxygen, you must wear a “reduc-
ing respirator” that lowers oxygen par-
tial pressure, or suffer -2 to DX due to
coughing and lung damage.
Very Dense (1.51+ atm.): As
“dense,” but a reducing respirator is
required if the air is more than 10%
oxygen. Usually quite hot from green-
house effects.
Superdense (10+ atm.): As “very
dense,” but the atmospheric pressure
is so great that it can actually crush
someone who is not native to it, unless
he has Pressure Support or an armored
suit that provides this advantage; see
Pressure (p. 435). Visitors to Venus, or
deep inside Jupiter, experience hun-
dreds of atmospheres of pressure! Such
atmospheres are often poisonous,
which presents a separate problem.
These rules assume you are native
to 1 atm. and can function normally
at 0.81-1.2 atm. If your native pres-
sure differs from 1 atm., multiply all
the pressure ranges above by your
native pressure in atm. For example,
if you’re native to 0.5 atm., a “dense”
atmosphere for you would be 0.61-
0.75 atm. and a “thin” one would be
0.26-0.4 atm.