Mustard gas, though technically not a gas and often called sulfur mustard by scholarly sources, is the prototypical substance of the sulfur-based family of cytotoxic and vesicant chemical warfare agents, which can form large blisters on exposed skin and in the lungs. It has a history of use as a blister-agent in warfare and, along with organoarsenic compounds such as Lewisite, is the most well-studied of such agents. [Source: Wikipedia 
]
