Definition of "breathable"

The Wards are open-topped, with skyscrapers rising from the superstructure. Towers are sealed against vacuum, as the breathable atmosphere envelope is only maintained to a height of about seven meters. The atmosphere is contained by the centrifugal force of rotation and a "membrane" of dense, colorless sulphur hexafluoride gas, held in place by carefully managed mass effect fields.

2008, BioWare, Mass Effect (Science Fiction), Redwood City: Electronic Arts, PC, scene: Citadel Station: Wards Codex entry