Short answers, pulled from the story.
The Organisation for the Prohibition of Chemical Weapons defines a chemical weapon as any compound or its precursor intended to cause death, injury, temporary incapacitation, or sensory irritation through chemical action. Delivery systems, whether filled or empty, are also classified as weapons. Precursor chemicals stored in bulk containers fall under the definition.
The Holocaust produced the largest death toll from chemical weapons in history. Nazi Germany used Zyklon B, a commercial hydrogen cyanide product, and carbon monoxide in gas chambers at extermination camps, causing the overwhelming majority of approximately three million deaths.
The Chemical Weapons Convention was concluded in 1993 and is administered by the OPCW, based in The Hague. It has 193 state parties. As of the most recent figures, 66,368 of 72,525 metric tonnes of declared chemical weapon stockpiles have been verified as destroyed.
The United States completed destruction of all its declared chemical weapons on the 7th of July, 2023. The disposal program was mandated by Congress in 1985, when the stockpile exceeded three million weapons totaling 31,000 tons.
World War I saw large-scale use of mustard gas, phosgene, and chlorine gas by most major powers. Military gas casualties across the war have been estimated between 500,000 and 1.3 million, with additional civilian casualties from production accidents and collateral exposure.
Unitary chemical weapons are already toxic in their existing state and require no mixing before use. Binary munitions contain two isolated, inert chemicals that only become lethal when combined, typically just before battlefield deployment. The majority of stockpiled chemical weapons worldwide have been unitary.