Mustard gas is a Poison gas that was first used in World War I. Nearly odorless, though yellow, it caused both internal and external bleeding, and killed over 4000 British soldiers alone in that war,  and wounded a great number more. Total casualties from all countries total over 100,000.  Adolf Hitler, who fought for Germany in this war, was a victim of mustard gas, and some have speculated that the great pain that he experienced led him not to use it as a weapon in World War II. Its chemical formula is C4H8Cl2S.
- http://www.bt.cdc.gov/agent/sulfurmustard/basics/facts.asp - Center for Disease control