Sezerfine gas, also known as Sezerfine concetrate, was a type of gas utilised by the Sontarans as a clone feed, serving a function analogous to amniotic fluid. Its composition was one part Bosteen, and two parts Probic 5.
In preparation of their 2009 invasion attempt of Earth the Sontarans manipulated humans to utilise ATMOS devices, seemingly a device to neutralise carbon dioxide emissions. However the true purpose of the ATMOS devices was to release Sezerfine gas into the atmosphere and thus turn the Earth into a clone world. The gas was eliminated from the Earth's atmosphere by the Tenth Doctor with Luke Rattigan's atmospheric converter. (TV: The Sontaran Stratagem / The Poison Sky, The Last Sontaran)