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Xyrene was a type of gas deadly to Humans, and existed aboard Federation starships in the 23rd century.
In the year 2268, the body of Captain James T. Kirk was possessed by Midgwin telepath Yarblis Geshkerroth. Geshkerroth sabotaged the life support system of Federation starship USS Enterprise so that it would release xyrene gas to kill the ship's crew. At one point, decks seven and eight were flooded with the gas. (TOS novel: Ghost-Walker)