Chemical Warfare was a necessity during world war one when involved in trench warfare. Poison gas is often mistaken for being first used by the Germans, but in fact it was first used by the French. Poison gas was feared throughout the war, it definitely was not something that you wanted to be faced with. The first gass attack was in 1914 during world war one by the french on german troops at the second battle of Ypers. Although the Germans were not the first to introduce poison gas, they definitely were the first to go into depth of experimentation and study of poison gas. 
In the beginning one of the methods the german soldiers used to protect themselves against poison gas was soaking cotton pads in urine. The ammonia from the urine, neutralized the chlorine from the gas and was held across the face until the soldiers were able to escape the area. Later on in 1915, gas masks were given to the soldiers (Gasschutzmaske). The masks had a rubber face piece and a removable filter. The masks were only able to filter out chlorine and tearing gases like xylyl Bromide, but unfortunately didnt work to well up against the deadly phosgene. 
There were three different forms/varieties that gas could come in:
Lachrymator
* This is very similar to mace and tearing gases used today. xylyl Bromide was a commonly used gas because of its simplicity to produce. This type of gas would inflame the lungs, eyes, mouth, and throat, it also caused temporary blindness. Gas masks protected soldiers very well from this type of gas.
Asphyxiant
* Asphyxiants are poisonous gasses such as ; chlorine, phosgene, and diphosgene. Chlorine, when it is exposed to moisture (found in eyes, mouth, throat, etc..) it forms a hydrochloric acid bond. Chlorine is deadly at the mix of 1.5000(gas/air) and phosgene is deadly at (1.10000) Diphosgene was even deadlier and could not be appropriately filtered out by a gas mask, which made it even worse. Diphosgene was very feared and was first used by the Germans.
Dichlorethylsulphide
* AKA (mustard gas) The one thing that differentiated mustard gas from the other gases was that it not only effected the respiratory system it also effected the skin, and would often cause the lungs to bleed. Mustand gas was an oily like substance and would cause awful blisters when came into contact with the skin and inside organs, eroding and eating away at the body. Mustard gas would also hang around long after it was exposed to an area, sometimes days, or even weeks. This was the most feared gas of all poison gas, it was not protected well by gas mask.
Casualties caused by poison gas is just unbelievable to me, i tend too usually forget about chemical warfare, but it is a major part of trench war fare and both of the world wars. Poison gas must have been the most awful experience especially mustard gas, I just cant even begin to imagine what it must have been like to experience such an awful trama.
The Evolution of Military War Craft. 