The first attack of gas in WW1 was in the first month it began and the French through a tear-gas grenade at the Germans and that was the start of gas being used in wars.
The Germans first used chlorine gas cylinders in April 1915 when it was employed against the French Army at Ypres.
Chlorine gas destroyed the respiratory organs of its victims and this led to a slow death by asphyxiation.
Mustard Gas
Mustard Gas (Yperite) was first used by the German Army in September 1917
The most lethal of all the poisonous chemicals used during the war, it was almost odorless and took 12 hours to take effect.
Mustard gas was produced in 1822 and was first used as a chemical warfare agent in WWI. Iraq also used large amounts of mustard gas in the war against Iran from 1979-1988.
A total of 91,198 soldiers from British Empire inc, Australia, France, United States, Italy, Russia, Germany, Austria-Hungary, and Others were killed by the gases