The "gas" is more like a fog of capsules. FM-200 is a common one. Basically it has a Fire suppression agent inside crystals which are blasted into the room by

The "gas" is more like a fog of capsules. FM-200 is a common one. Basically it has a Fire suppression agent inside crystals which are blasted into the room by compressed air. These crystals melt when they get over a certain temperature and therefore won't kill you; however, breathing that in isn't really pleasant. Source: I've been in an FM-200 discharge