Hexachromite gas was a smoky gas used to seal underwater structures. It was used by humans in the late 21st century. 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 kill them, causing the body to break down into a green slime. In 2084, the Fifth Doctor fed the gas into the ventilation system of Sea Base 4 to kill the Silurians and Sea Devils within. (TV: Warriors of the Deep)