Mustard gas is a chemical warfare agent of great toxic effect, used in the manufacture of gas bombs. Mustard gas agent was first applied in World War I.

Since the Ypres gas war, decision makers and commanders on both sides of the war became keen on chemical warfare. The war promoted the development of chemical weapons, and a variety of toxic agents appeared in World War I. In addition to chlorine gas, mustard gas appeared, which was a poisonous devil on the battlefield that brutalized living beings. Mustard gas is a liquid vesicant agent, invading the human body will cause systemic poisoning. In World War I, the German army first manufactured and used mustard gas agent, made of agent bombs. Different agent bombs with different code names, "yellow cross" for the vesicant agent bomb, "green cross" for the asphyxiating agent bomb, "blue cross" for the sneezing agent bomb.

On July 12, 1917, disaster struck again at British positions on the Ypres battlefield. The Germans fired a large number of mustard gas agent shells to the British positions, the next 10 days, the German army **** fired more than 1 million rounds of "Yellow Cross" agent shells, about 2,500 tons of mustard gas. The number of British troops who were poisoned was 15,000, with 500 deaths. Due to the British troops by the "Yellow Cross" agent bomb attack, the original plan of attack had to be postponed.

According to statistics, in the First World War, 12,000 tons of mustard gas were used in the war; the number of casualties due to gas reached 1.3 million, 88.9% of which were due to mustard gas poisoning.