The substance was widely used in chemical warfare in World War One - and causes blisters on the skin and lungs.
That's a bit of an understatement. The stuff kills you, and if not, blinds you. My grandfather was gassed during WWI, and my father told how he would cough his lungs up for half an hour when he went upstairs to a colder bedroom. Even after some forty-five years. And he was one of the lucky (?) ones.