Mustard Gas (Yperite) was first used by the German Army in September 1917. The most lethal of all the poisonous chemicals used during the war, it was almost odourless and took twelve hours to take effect. Yperite was so powerful that only small amounts had to be added to high explosive shells to be effective. Once in the soil, mustard gas remained active for several weeks.

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 and attacked the bronchial tubes, stripping off the mucous membrane. This was extremely painful and most soldiers had to be strapped to their beds. It usually took a person four or five weeks to die of mustard gas poisoning. One nurse, Vera Brittain, wrote: "I wish those people who talk about going on with this war whatever it costs could see the soldiers suffering from mustard gas poisoning. Great mustard-coloured blisters, blind eyes, all sticky and stuck together, always fighting for breath, with voices a mere whisper, saying that their throats are closing and they know they will choke."

British Gas Casualties: 1914-18
Deaths - Non-Fatal
Chlorine 1,976 - 164,457
Mustard Gas 4,086 - 16,526