Mustard gas AKA sulfur mustard is a fine mist of liquid droplets frequently used as a chemical warfare agent due to its ability to interfere with biological processes, forming blisters upon exposed skin and within the lungs, frequently resulting in illness leading to death. Mustard gas is actually several compounds containing the chemical SCH₂CH[/sub]2[/sub]Cl, with the typical version being called bis(2-chloroethyl) sulfide.

Mustard gas was first used in warfare on July 12, 1917, during World War I, when the German army gassed British and Canadian soldiers near Ypres, Belgium. Since then, it has been used in many wars and other conflicts, frequently against people who can't retaliate in return. In accordance with the Chemical Weapons Convention in 1997, signatory countries can't produce or stockpile stores of mustard gas. Since then, the vast majority of stockpiles have been destroyed or disposed of.

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Mustard gas (Wikipedia)