Toxic atmospheres cause Fortitude saves (DC 15 mild to 20 lethal) each minute or take 1d4 CON damage and gain Sickened/Exhausted.
Toxic atmospheres are areas filled with poisonous gases -- a chemical spill, a Swamp planet, or a venting reactor. The zone is divided into tiers based on toxicity: mild (DC 15), moderate (DC 18), and lethal (DC 20). At each interval (1 minute for ambient zones, 1 round for direct exposure), characters inside must make a Fortitude save against the zone's DC. A failed save deals 1d4 Constitution damage; mild zones impose Sickened, while moderate and lethal zones impose Exhausted. Characters wearing filter masks or sealed suits gain +4 resistance. Constructs are immune to toxic effects.