<P> The first instance of large - scale use of gas as a weapon was on 31 January 1915, when Germany fired 18,000 artillery shells containing liquid xylyl bromide tear gas on Russian positions on the Rawka River, west of Warsaw during the Battle of Bolimov . Instead of vaporizing, the chemical froze and failed to have the desired effect . </P> <P> The first killing agent used by the German military was chlorine . Chlorine is a powerful irritant that can inflict damage to the eyes, nose, throat and lungs . At high concentrations and prolonged exposure it can cause death by asphyxiation . German chemical companies BASF, Hoechst and Bayer (which formed the IG Farben conglomerate in 1925) had been making chlorine as a by - product of their dye manufacturing . In cooperation with Fritz Haber of the Kaiser Wilhelm Institute for Chemistry in Berlin, they began developing methods of discharging chlorine gas against enemy trenches . </P> <P> According to the fieldpost letter of Major Karl von Zingler, the first chlorine gas attack by German forces took place before 2 January 1915: "In other war theatres it does not go better and it has been said that our Chlorine is very effective . 140 English officers have been killed . This is a horrible weapon ...". </P> <P> By 22 April 1915, the German Army had 168 tons of chlorine deployed in 5,730 cylinders from Langemark--Poelkapelle, north of Ypres . At 17: 30, in a slight easterly breeze, the gas was released, forming a grey - green cloud that drifted across positions held by French Colonial troops from Martinique, as well as the 1st Tirailleurs and the 2nd Zouaves from Algeria . Faced with an unfamiliar threat these troops broke ranks, abandoning their trenches and creating an 8,000 - yard (7 km) gap in the Allied line . The German infantry were also wary of the gas and, lacking reinforcements, failed to exploit the break before the 1st Canadian Division and assorted French troops reformed the line in scattered, hastily prepared positions 1,000--3,000 yards (910--2,740 m) apart . The Entente governments claimed the attack was a flagrant violation of international law but Germany argued that the Hague treaty had only banned chemical shells, rather than the use of gas projectors . </P>

When was chlorine gas first used in ww1