Mustard gas

Mustard gas was an offensive weapon used on Earth in at least World War I. During World War II, Colonel Schott, a veteran of that war, recalled the British use of the gas against the Germans at Ypres. (PROSE: Illegal Alien) The Eighth Doctor breathed in mustard gas in No Man's Land. (AUDIO: The Great War)

The 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 compared the Breath of forever to mustard gas. (AUDIO: Time's Horizon)