Hexachromite gas

Hexachromite gas is smokey gas used to seal underwater structures. It was used by humans in 2084. It has no effect on mammals such as humans, Gallifreyans or Trions, but is poisonous to reptiles and marine life.

It can cause pain to Silurians and Sea Devils when breathed in and will eventually lead to death, causing the body to break down from the inside into a green slime. Silurians have a higher immunity to it than Sea Devils but will succumb eventually. (DW: Warriors of the Deep)