A chemical oxygen generator is a evice that releases oxygen created by a chemical reaction. The oxygen source is usually an inorganic superoxide, chlorate or perchlorate. A promising group of oxygen sources are ozonides. The generators are usually ignited mechanically, by a firing pin, and the chemical reaction is usually exothermic, making the generator a potential fire hazard. Potassium superoxide was used as an oxygen source on early manned USSR space missions, for firefighters and mine rescue.