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.
Gas! GAS! Quick, boys! — An ecstasy of fumbling,
Fitting the clumsy helmets just in time;
But someone still was yelling out and stumbling,
And flound’ring like a man in fire or lime …
Dim, through the misty panes and thick green light,
As under a green sea, I saw him drowning.
In all my dreams, before my helpless sight,
He plunges at me, guttering, choking, drowning.
— Wilfred Owen, “Dulce et Decorum est”, 1917