Mustard gas deployment near Ypres in 1917

During World War I, the introduction of mustard gas, also known as sulfur mustard or iprit, fundamentally altered chemical warfare. While often referred to as a gas, it is actually a volatile oily liquid that can disperse in the air and persist on the ground, clothing, and military equipment.

Germany first deployed the weapon against British positions near Ypres, Belgium, during the night of July 12–13, 1917, using artillery shells marked with a yellow cross. Unlike previous chemical agents, mustard gas is characterized by a delayed effect; soldiers often do not realize they have been exposed until hours later when painful blisters, eye inflammation, and respiratory damage appear. While gas masks can protect the lungs, they do not provide full protection for the skin.

Beyond its lethal capabilities, the substance was used to incapacitate large groups of soldiers, removing them from the battlefield. According to data from the Organisation for the Prohibition of Chemical Weapons, chemical agents used during the First World War caused over one million casualties and more than 90,000 deaths. The persistence of mustard gas in the environment turned entire sectors of the front into hazardous zones.