Short answers, pulled from the story.
The Organisation for the Prohibition of Chemical Weapons defines a chemical weapon as any chemical compound intended as a weapon or its precursor that can cause death, injury, temporary incapacitation or sensory irritation through its chemical action. This definition includes lethal examples such as nerve agents and blister agents.
Large-scale use of chemical weapons occurred between 1914 and 1918 during World War I when mustard gas, phosgene gas, and chlorine were employed to break the stalemate of trench warfare. Estimates for military gas casualties from this period range from 500,000 to 1.3 million people.
The Holocaust remains the deadliest use of poison gas in history with an overwhelming majority of some three million deaths resulting from vast quantities of Zyklon B gas and carbon monoxide used in gas chambers. Peter Longerich documented this persecution and murder of the Jews in his 2010 book published by Oxford University Press.
The Geneva Protocol entered into force on the 8th of February 1925 prohibiting the use of chemical and biological weapons in international armed conflicts. Ukraine became the newest signatory acceding the 7th of August 2003.
Sixty-six thousand three hundred sixty-eight metric tonnes representing 92% of chemical weapon stockpiles have been verified as destroyed under the 1993 Chemical Weapons Convention. This agreement is administered by the Organisation for the Prohibition of Chemical Weapons based in The Hague.