Mustard gas was used in Flanders, near the town of Ypres, on 12 July 1917. This was its first reported use in modern warfare, and one of its alternative names is Yprite. The chemical was first synthesised much earlier, certainly in 1860 by Frederick Guthrie, and maybe earlier still according to different sources.
The main active ingredient of mustard gas is the chemical designated 1,1 thiobis(2 - chloroethane), "thio" indicating the presence of sulfur in the molecule, which also contains two atoms of chlorine. The mustard gas used in World War I was not the pure compound and was actually a thick, viscous liquid with a yellowy colour, hence the name mustard gas. Impure, it also had the odour of mustard or garlic, so at least you knew it was coming. The pure compound is both odourless and colourless, so would probably have caused even more damage if it could have been used. The liquid had to be dispersed in the form of an aerosol in order for it to be used as a weapon. The easiest way to do this of course was to put it into a shell and explode it amongst the enemy.
Mustard gas is a vesicant. Once in contact with the skin it causes reddening and then blisters. Any moisture on the skin would cause a hydrolysis, a term for specific reaction where a compound reacts with and is broken up by water. Unfortunately one of the major products of this reaction with mustard gas is HCl, Hydrochloric acid, which would further burn the skin causing agonising pain. The effect of mustard gas was not to instantly kill troops but to cause panic and disorientation, to prepare for an infantry attack. The clothing and protective masks of the time had little effect as the gas just seeped through them.
The eyes, because they are wet, were particularly vulnerable, and victims were often blinded. At the time the only relief was to wash the effected area, but this would require lots of clean water, both to wash away the gas and then the products created.
Once the gas was inhaled of course the same types of reactions would occur but this time in the lungs, or in the stomach if some of the gas was swallowed. The linings of the lungs would blister and bleed, breathing would become impossible. Victims would cough up blood and the linings of the nasal cavity and of the mouth and throat would also blister and burn.
Although it reacts readily with water it is not very soluble so smaller amounts of gas could be breathed in without an immediate reaction and so symptoms could materialise hours after inhalation. The chemical would persist in the victims clothing, which needed to be removed quickly, and then the victim washed repeatedly. Obviously not an option in the heat of battle.
Survivors would be treated for the effects of burning and blistering to the skin. Internal damage would have to sort itself out, or not. Then the victim would have to live with the knowledge that cancer was a possibility in latter life. No antidote for mustard gas exists, although mild bleach solutions were used to neutralize the gas more quickly, as doctors sought ways to combat this new menace.
The liquid form of mustard gas could persist in the trenches for days dependent on the weather conditions, it was not often that really heavy rain would be welcomed in the trenches. If it was not exposed to moisture the chemical could persist for years especially inside shell casings. Although now banned under The Chemical Weapons Convention ( CWC ), mustard gas and its close chemical cousins have found uses in cancer research. Nitrogen mustard gas, similar to the sulfur version but with a nitrogen atom in place of the sulfur atom, has been used in chemotherapy in the cure of Hodgkin's Disease.