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 Rani employed it as a defensive mechanism against those who meant to break in her TARDIS. The Sixth Doctor and Peri Brown wore masks to avoid its effects. (TV: The Mark of the Rani)