Germany first used chlorine gas as a weapon in World War I on April 22, 1915 in the Second Battle of Ypres. Described by soldiers as having a distinctive odour—a mixture of pepper and pineapple. It had a metallic taste and stung the back of the throat and chest. Chlorine gas destroyed the respiratory tract of its victims which led to a slow death by asphyxiation.

Chlorine was, however, less effective as a weapon than the Germans had hoped, particularly when simple counter-measures were introduced. The gas produced a visible greenish cloud and had a strong odour, making it easily detectable. It was water-soluble, so even the simple act of covering the nose and mouth with a damp cloth was somewhat effective at reducing its effect.

The use of poison gas in World War I was a major military innovation. The gases used ranged from tear gas to disabling chemicals such as mustard gas as well as killing agents such as phosgene. The killing capacity of gas was limited—only 3 percent of combat deaths were due to gas—however, the proportion of non-fatal casualties was high and gas remained one of the soldiers’ greatest fears.

The French were the first to employ gas in August 1914 when they fired tear-gas grenades on the Germans. This type of gas was used an irritant, not a killing agent. Tear gas was designed to incapacitate an enemy and render them incapable of defending their positions.

Phosgene gas was the first chemical weapon used by Germany during World War I and was the major cause of chemical death. When first used against British troops in December 1915, 88 tons were released, causing 1,069 casualties, including 120 deaths. Phosgene was colourless so difficult to detect and had an odour likened to mouldy hay. Its effect was often delayed, with seemingly healthy soldiers displaying symptoms of phosgene gas poisoning up to 48 hours after inhalation.

Its use as a major chemical weapon quickly diminished once mustard gas was developed and introduced to the battlefield.

Mustard gas was first used effectively in 1915 by the German army against Canadian soldiers and later also against the French. An oily chemical liquid, which when evaporated forms a poison gas, it was dispersed as an aerosol. The yellow-brown colour and distinctive mustard-like odour gave it its name.

It burns the skin and damages the respiratory tract, causing serious blisters (both internal and external) several hours after exposure. It was used primarily to harass and disable the enemy and to pollute the battlefield, but in high concentrations proved fatal to those exposed. Protection against the gas proved more difficult than against either chlorine or phosgene gas and it was considered a very effective chemical weapon.