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.
Though poison gas was used in WWI, it received numerous technological advancements toward WWII. More effective gases such as "Mustard Gas" and several "Nerve" gases were developed. A few seconds after inhaling these gases the victim would start convulsing and eventually die. Poison Gas was rarely used against enemies due to the fear that the enemy would retaliate with a more virulent and devastating gas.
The first time that poison gas was ever used was in 1915 during the Second Battle of Ypres. During the morning of April 22, Germans poured a heavy bombardment around Ypres. The effects of the chlorine gas were severe. Within seconds of inhaling the vapour, it destroyed the victim's respiratory organs, bringing on choking attacks.
The experimental use of equine goggles against the effects of effects of chlorine and vesicatory gases. Unfortunately, they were not used routinely by the Allies - possibly because they tended to fog up.
World War I Photograph, “Mustard Gas Victims.” The extensive bandages on these wounded Canadian soldiers may indicate that they have suffered the effects of flame or mustard gas. Mustard gas burned the lungs, but also caused serious external blisters and disfigurement. Source: Canadian War Museum - Musée canadien de la guerre
The first poison gas, chlorine,came on 22 April 1915, at the start of the Second Battle of Ypres. Around 5 pm, sentries posted among the French and Algerian troops noticed a curious yellow-green cloud drifting slowly towards their line. It signalled the first use of chlorine gas on the battlefield. The effects were severe. Within seconds of inhaling its vapour it destroyed the victim's respiratory organs, bringing on choking attacks. (For a memoir of the first gas attack click here.)
Mustard Gas Mustard Gas (Yperite) was first used by the German Army in September 1917. It was one of the most lethal of all the poisonous chemicals used during the war. It was almost odourless and took twelve hours to take effect. Yperite was so powerful that only small amounts had to be added to high explosive shells to be effective. Once in the soil, mustard gas remained active for several weeks.