"who are already being issued with gas masks"
German WWI Gas Mask
Public DomainGerman WWI Gas Mask - Credit: Daderot

Gas was first used by the French in August 1914, but from January 1915 the Germans began deploying gas on both fronts.  Different chemicals were used: tear gas, poison gas and suffocating gas.  The most common poisons were mustard gas, chlorine and phosgene.

US WWI Gask Mask
Creative Commons Attribution Share AlikeUS WWI Gask Mask - Credit: Historicair

Over time, both the Axis powers and the Allies designed various types of gas masks. The first masks issued were just cotton cloths that one held over the nose and mouth. Then came fabric masks made of chemical absorbing cloth that fitted over the whole head. The most advanced mask of WWI, the canister gas mask, consisted of a mask attached to a canister that held chemical absorbent materials such as sodium hyposulphate and glycerine. 

Even with advances in gas mask technology, poison gas easily penetrated gas masks and killed soldiers.  Many of those who survived suffered weakened lungs and died soon after the war of respiratory problems.

WWI Gas Warfare