Metals have been used as instruments of murder. Arsenic is perhaps more rightly classified as a metalloid, but it is consistently the single substance most commonly thought of as a poison. Metals have also been used in warfare as chemical weapons. Again, arsenic was the primary component of the spray known as Lewisite that was used by the British during trench warfare in World War I. Exposure produced severe edema of the eyelids, gastrointestinal irritation, and both central and peripheral neuropathies. The first antidote to heavy metal poisoning, and the basis for chelation therapy today, was British Anti-Lewisite (BAL, or dimercaprol), a large molecule with sulfhydryl groups that bind arsenic, as well as other metals, to form stable covalent bonds that can then be excreted by the body. BAL was developed by the Germans during World War II in anticipation of a reinitiation of gas warfare as had been waged earlier in the century.