Chlorine was the first chemical to be specifically and effectively used as a lethal agent in World War I, although "non-lethal" chemical harassing agents had been used from early on in the war.
K-Stoff was introduced by the Germans in June 1915 as a more lethal replacement for chlorine. A low-boiling liquid (boiling point 1 degree of Celcius), it was loaded into artillery shells, hence was more convenient to use in combat, and could be delivered independent of the prevailing wind direction. It was manufactured as a mixture of chloromethyl chloroformate and dichloromethyl chloroformate. This agent is about as twice toxic as chlorine.
Phosgene and Diphosgene
Phosgene and Diphosgene have a toxicity of six times more than chlorine and became the most heavily used nonpersistent lethal agents of the war: phosgene by the Allies, diphosgene by the Germans. Diphosgene, or trichloromethyl chloroformate, the fully chlorinated analog of K-Stoff, is less volatile than phosgene, but readily breaks down in vivo or when heated to give 2 mols of phosgene.
Chloropicrin was introduced by the Russians in August 1916 as an agent that would penetrate the protective masks then being used. It is said to be comparable in toxicity to chlorine, although another source reports it to be intermediate in toxicity between chlorine and phosgene.
It was first used by the French in October 1916 and was not an important agent in World War I. However, it was standardized and stocked by the United States during World War II as a quick-acting, nonpersistent gas that was supposedly able to penetrate the 1941-1942 model German and Japanese masks. Hydrogen cyanide (HCN) was used in considerable quantities by the French in World War I, with apparently little success.
Mustard was the most feared chemical agent to emerge from World War I. Introduced by the Germans in July 1917, it was the most effective casualty-producing agent of the war. Eventually, it was also manufactured by the Allies, but not until the very end of the war. It was used by the Italians against Ethiopians in 1935-1936, and by the Japanese against Chinese in 1937-1942.
Ethyldichloroarsine and phenyldichloroarsine
Both agents are two World War I arsenic derivatives which were also designed by the Germans to either circumvent or penetrate the Allies' masks. However, these agents were introduced rather late in the war (March 1918 and September 1917) and saw only limited use, probably because mustard was already being effective used at that time.
Nitrogen mustard and its monethyl and monomethyl analogs are liquids vesicants that were evaluated shortly before and during World War II. Nitrogen mustard was stockpile Germany during the war, but neither it nor its analogs are known to have been used in combat or to exist in any current stockpile.
Noyes, Robert. 1996. Chemical Weapons Destruction and Explosive Waste: Unexploded Ordnance Remediation. Noyes Publications: USA.