Mustard gas, also known as dichlorodiethyl sulphide, was an offensive weapon used on Earth in at least World War I. (PROSE: The Mark of the Rani) It was produced from potassium sulfide, chloroethanol and phosphorus trichloride. (AUDIO: The Great War)
Colonel Schott, a veteran of that war, recalled the British use of the gas against the Germans at Ypres. (PROSE: Illegal Alien) The gas was also used in an attack at Verdun, blinding Captain Parkinson. (PROSE: Remembrance of the Daleks) The Eighth Doctor breathed in mustard gas in No Man's Land. (AUDIO: The Great War)
The First Rani employed it as a defensive mechanism against those who meant to break into her TARDIS. (TV: The Mark of the Rani)
The Eighth Doctor's companion Molly O'Sullivan, a VAT from World War I, compared the breath of forever to mustard gas. (AUDIO: Time's Horizon)