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Hexachromite gas was a smoky 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 was painful to Silurians and Sea Devils to breathe and would eventually lead to death, causing the body to break down into a green slime. Silurians had more resistance to it than Sea Devils but would succumb eventually. (DW: Warriors of the Deep)