Hexachromite gas

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

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