Atmospheric Pressure

Atmospheric Pressure Regardless of its composition, an atmosphere may be difficult or impossible to breathe if its pressure is wrong. We measure air pressure in "atmospheres" (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).

Very Thin (up to 0.5 atm.): The air is too thin to breathe. Earth's atmosphere becomes "very thin" above 20,000' (6,000 m). If you lack protection (e.g., the Doesn't Breathe advantage, or a respirator and oxygen tanks), you suffocate – see Suffocation. Vision rolls are at -2 without eye protection.

Thin (0.51-0.8 atm.): Earth's atmosphere is "thin" between 6,000' and 20,000' (1,800 to 6,000 m). Thin air is breathable if oxygen is present in Earthlike percentages, 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, anyone who breathes thin air for an hour or more must check for "altitude sickness." Make a daily HT roll at +4. Critical success means acclimatization – 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. 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 pressure suit. If the air contains more than 50% oxygen, you must wear a "reducing respirator" that lowers oxygen partial 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 greenhouse 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. Visitors to Venus, or deep inside Jupiter, experience hundreds 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 pressure 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.