Chemical weapons in World War I
Mustard Gas: First used by the Germans in 1917.  It was the most effective and most publicized gas in  the war.  It caused huge, painful yellow blisters on contact with skin.  Few victims died.  Gas masks were ineffective against this heavy and dense mist.  It sank to the bottom of trenches and into cracks and crevices and stayed there for weeks or even months.