In the 19th century Great Britain used chemical weapons at war[citation needed] and others planned to use it[citation needed]. In the Hague Conventions of 1899 and 1907 the use of dangerous chemical agents were outlawed. In spite of this, the First World War saw large-scale chemical warfare. France used teargas in 1914, but the first large-scale successful deployment of chemical weapons was by the German Empire in Ypres, Kingdom of Belgium in 1915, when chlorine gas was released as part of a German attack at the Battle of Gravenstafel. Following this, a chemical arms race began, including Great Britain, Russia, Austria-Hungary, USA and Italy. This resulted in the development of a range of horrific chemicals affecting lungs, skin, or eyes. Some were intended to be lethal on the battle field, like hydrogen cyanide, and efficient methods of deploying agents were invented. At least 124,000 tons was produced during the war. In 1918 about one grenade out of three was filled with dangerous chemical agents. Around 1% of the fatalities and 4% of woundings of the Great War can be attributed to the use of gas, although the psychological affect on troops may have had a much greater effect. As protective equipment developed, the technology to destroy such equipment also became a part of the arms race.
The Treaty of Versailles included some provisions that banned Germany from either manufacturing or importing chemical weapons. Similar treaties banned the First Austrian Republic, the Kingdom of Bulgaria, and the Kingdom of Hungary from chemical weapons, all belonging to the losing side, the Central powers. Russian bolsheviks and Britain continued the use of chemical weapons in the Russian Civil War and possibly in the Middle East in 1920.
Three years after World War I, the Allies wanted to reaffirm the Treaty of Versailles, and in 1922 the United States introduced the treaty on the Use of Submarines and Noxious Gases in Warfare, known as the Washington Treaty.[5] The four of the war victors, United States, United Kingdom, Italy and Japan, gave consent for ratification but it failed to enter into force as the French Third Republic objected to the submarine provisions of the treaty and thus the treaty failed.[5]
At the 1925 Geneva Conference for the Supervision of the International Traffic in Arms the French suggested a protocol for non-use of poisonous gases. The Second Polish Republic suggested the addition of bacteriological weapons. It was signed on 17 June.[6]

But the US still used it (agent orange) in Vietnam war that produced maldeveloped human being:

I think it is banned not because of the kill but of the consequential effects.