Mustard gas
From Tardis Wiki, the free Doctor Who reference
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)
