There were three main types of poisonous gas used in trench warfare: chlorine, phosgene and mustard gas. Although it could kill, chlorine gas was easy to detect. Mustard gas was first used by the Germans in 1917 and it was incredibly effective. Although it wasn't as fatal as phosgene, mustard gas could linger over the battlefields and cause horrific burns. Phosgene however was a lot more powerful and it was difficult to detect - this became the main killing gas of WWI.