Hexachromite gas

Hexachromite gas is smokey gas used by the humans of 2084 to seal their ships. Though it has no effect on such creatures as Humans, Gallifreyans or Trions, it is very dangerous to reptiles like Silurians and Sea Devils. It can cause pain to Silurians and Sea Devils when breathed in and will eventually lead to death. After death, the body breaks down from the inside into a green slime. The Silurians have a higher immunity to it then the Sea Devils. (DW: Warriors of the Deep)