Mustard gas

Mustard gas, also known as sulfur mustard, is a chemical warfare agent used during the First World War. Yellow Cross, another chemical warfare agent, is based on mustard gas and so called because of the marking on the shells. These are a vesicant (blister agent) poison gas, one effect of which was to blister the mouth, armpits and face, and affect the eyes. It was first used in July 1917 by the Germans, causing nausea and a succession of debilitating symptoms including intense itching and painful blisters filled with yellow fluid. The gas hung low on the ground, often remaining inert until the sun's warmth caused the gas to rise.

See also Gas and the detailed Wikipedia article.