It was a vesicant that was introduced by Germany in July 1917 prior to the Third Battle of Ypres. The Germans marked their shells yellow for mustard gas and green for chlorine and phosgene; hence they called the new gas Yellow Cross. The skin of victims of mustard gas blistered, the eyes became very sore and they began to vomit. Mustard gas caused internal and external bleeding. mustard gas killed 4,086 non-fatal16,526