Respirators are required for humans to breathe the toxic air on Euphrosyne, otherwise one might become feral. When the air is filtered, the resulting toxic soot is often collected by members of the Clans. This byproduct, known as feralite, is consumed as a dangerous, hallucinogenic ritual drug.
indoor environments
Most post-war buildings on the planet are equipped with airlocks and air filters. Because of this, it is normal for people to remove their respirators inside to eat, drink, and talk safely. Pre-war buildings, however, never have these features, as the atmosphere was mostly safe before the Great War except in specific industrial areas.
biological adaptation
Unlike humans, gorillas and other local animals have adapted to the toxic atmosphere over time and do not need to wear respirators.