Gas was a very effective way of attacking the enemy without direct contact, and attacks were the thing soldiers in the trenches feared over anything. This manual originally published by the War Office in February 1917, covers topics including the nature of gas: attacks, types, projectiles, personal anti-gas equipment & general precautions & observations.
Chlorine gas was first used by the Germans 31st of January 1915 It was a highly visible cloud of green. It damaged the eyes, throat, nose, ears and lungs. Death was caused by asphyxiation.
By 1917 the Germans were using Mustard Gas: the most effective and most publicised gas in the Great War, causing painful yellow blisters on contact with skin. Gas masks were ineffective against this heavy and dense mist that sank to the bottom of trenches and into cracks and crevices and stayed there for weeks or even months.