Thick

A nonacclimated creature operating in a thick atmosphere treats it as somewhat harmful, due to the extra chemical compounds in the air and the increased atmospheric pressure. Every hour, such a creature must succeed at a Fortitude save (DC = 15 + 1 per previous check) or become sickened. This condition ends when the creature returns to a normal atmosphere. Conversely, the increased weight of the air grants a +4 circumstance bonus to Acrobatics checks to fly or Piloting checks to keep an aircraft in flight.

Severely thick atmospheres are far more dangerous. Every minute, a creature in such an atmosphere must succeed at a Fortitude save (DC = 15 + 1 per previous check) or begin to suffocate (see Suffocation and Drowning on page 404) as its lungs cease coping with the density of the oxygen inhaled and lose the strength to keep pumping air into its bloodstream.