Hexachromite Gas
From Multiversal Omnipedia
Hexachromite gas was a gas used by the staff of Sea Base 4 as a sealant. Although it was harmless to humans, it was lethal to creatures like the Silurians and Sea Devils. When Icthar tried to provoke a nuclear war that would wipe out the human race by taking over Sea Base 4 and using its missile arsenal, the Fifth Doctor was forced to use the gas to kill the Silurians and their allies in order to save humanity.