Methoamyline was a type of gas which was deadly to Humans, but was breathable to some other species.
History and specifics[]
Denovians breathed methoamyline.
In the year 2268, when the Federation starship USS Enterprise took Denovian diplomatic envoys to the planet Giermos, they required canisters of methoamyline gas to breathe while aboard the ship.
Later that year, Midgwin telepath Yarblis Geshkerroth hid a leftover tank from that mission in the air duct to Helen Gordon's quarters with the intent of killing her. Geshkerroth had possessed the body of Captain James T. Kirk and knew of the mission to Giermos from Kirk's memories. The tank was discovered, however, and Gordon survived the murder attempt. (TOS novel: Ghost-Walker)