In the World War One poem "Dulce et Decorum Est," the poet Wilfred Owen exposes the brutal reality of the war, and to this end he describes in gruesome, horrific detail the pain of one soldier who inhales poisonous gas. Once the soldier inhales the gas, he is described as "flound'ring like a man in fire or lime."
The above simile makes it clear that after being exposed to the gas the soldier feels an intense, unbearable burning sensation, as if he has been set on fire. This implies that the gas was probably mustard gas, as mustard gas is known to burn and blister the skin and lungs of those unfortunate enough to be exposed to it. This burning and blistering must have felt very much like the sensation of being set on fire.
The use of poisonous gas in World War One is thought to have killed approximately 91,000 people. There were other poisonous gases besides mustard gas, but it is thought that up to 85% of those 91,000 deaths were attributable specifically to mustard gas. This also makes it likely, therefore, that the gas described in the poem is mustard gas.
Mustard gas was used for the first time in World War One in July 1917, in Ypres, Belgium, deployed by the Germans against the British. In 1925, the Geneva Gas Protocol, signed at the time by most of the countries in the world, made the use of any poisonous gases in war, including mustard gas, illegal.