Asphyxiation Gas was first introduced by the Germans on April 22nd, 1915. It was used on an attack on Algerian and Zouave troops. The troops were taken completely by surprised, and broke before it. On the two following days, an attack was made on the Canadians and Americans in Ypres, under the cover of gas. Although no respirators were available, and the troops suffered heavy losses, they still held their ground. The gas has never been recovered or analysed, but speculation believes it to have probably been Chlorine gas. Chlorine was chosen because of how fast and hard it acts. It could put a man out of action when inhaled in a strength of only 1 in 10,000, whereas sulfur is only effective in a concentration four times as great. Chlorine is also heavier than air, so the yellow gas would sink into the trenches and craters. It can also be rapidly manufactured, into small cylinders, or canisters.
Chlorine gas is a asphyxiation gas, meaning it makes the victim have trouble or completely stop being able to breathe. It also causes a burning pain in the throat and eyes. The person slowly begins to have trouble breathing until they pass out, usually succumbing to the weakness and lowering to the ground. Which that doesn't help, since Chlorine Gas is heavier than air and floats closer to the ground.