Inert Gas

Inert gas atmospheres aren’t hostile or poisonous, but they’re unbreathable by humans. If the planet’s climate is otherwise tolerable, the natives might be able to live and work outdoors without anything more than an air tank and face mask. Some degree of technical sophistication is going to be necessary for humanity to survive, and large supplies of oxygen are going to have to be extracted from some local source. “Air mines” might exist to replace the oxygen lost during human use, or local power plants might need to be kept running full blast in order to crack water molecules into hydrogen and oxygen.

On worlds with an unbreathable atmosphere, control of the air supply makes it much harder for locals to rebel against their leaders. Without the equipment and power to generate their own air supplies, would-be rebels are entirely at the mercy of those who control the oxygen. Most rulers pick up on this fact rapidly, and some worlds with unbreathable atmospheres might even make it a crime to own illicit oxygen generation equipment.