These agents inhibit the enzyme cytochrome oxidase (COX) preventing the transfer of oxygen from blood to the cells. Materials include cyanides, halogenated cyanides, and hydrogen sulfide. They are first generation warfare agents that were used during World War I. They were the first systemic agents introduced in that war. They are well-known industrial materials that were readily available at that time; most still have commercial value. Hydrogen cyanide and cyanogen chloride are listed in Schedule 3 of the Chemical Weapons Convention and are the only COX inhibiting blood agents specifically included in the convention. COX inhibiting blood agents are relatively easy to acquire or manufacture and to disperse. For information on some of the chemicals used tomanufacture these blood agents, see the Component section (C07-C) following information on the individual agents. Blood agents have been stockpiled by most countries that have pursued a chemical

weapons program, and have been used a number of times on the battlefield. Although this class of agents is considered obsolete on the modern battlefield, several of these agents are still considered a significant threat as potential improvised weapons that could be utilized in urban warfare. COX inhibiting blood agents have also been used by terrorists.