A vesicant (i.e., an agent that produces vesicles or blisters) was first used as a chemical weapon on the battlefields of World War I; that same vesicant-- sulfur mustard--is still considered a major chemical agent. In the intervening years between World War I and today, there have been a number of recorded and suspected incidents of mustard use, culminating with the Iran-Iraq War in the 1980s. The military has considered vesicants to be major chemical warfare agents since 1917. Mustard, however, is the only vesicant known to have been used on the battlefield. Mustard and Lewisite, in much smaller amounts, are known to be in the stockpiles of other countries. Mustard was used on a large scale in World War I, causing a great number of casualties; it was also used during the Iran-Iraq War. Data from the Iran-Iraq War are scanty; however, data from World War I indicate that more than 95% of mustard casualties survived but most required lengthy hospitalizations. If mustard is ever used again, military medical personnel must be prepared to accept and care for large numbers of casualties, who will require long-term hospitalization.
Textbook of Military Medicine: Medical Aspects of Chemical and Biological Warfare, p.197-228