Poison gas has been used in warfare since ancient times, however World War One was the first conflict where chemical warfare was deployed on a large scale. Chemical warfare was especially devastating in World War I due to the use of trenches, with trenches trapping poison gas. However, the use of poison gas was hazardous, as the delivery of the gas was heavily dependent of the wind; leakages or bad wind conditions often resulted in high casualties on the side using the weapons. The name Poison Gas is often misleading as many of the substances used were liquids that formed a vapour.
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Tear gases used in the war were: Xylyl Bromide, Benzyl Bromide, Chloropicrin, Ethyl Iodoacetate, Bromoacetone, Monobromethyl ethyl ketone, Acrolein and Benzyl Chloride.