Which chemical agent is specifically classified as a blood agent?

Cyanide is classified as a blood agent because it interferes with the body's ability to utilize oxygen at the cellular level. Blood agents work primarily by being absorbed into the bloodstream and then preventing cells from utilizing oxygen effectively. This leads to asphyxiation at the cellular level, which can rapidly result in unconsciousness and death if not treated immediately.

In contrast, the other substances listed serve different purposes and have different mechanisms of action. Sarin is a nerve agent that disrupts the normal functioning of the nervous system. Chlorine and phosgene are chemical agents classified as choking agents or pulmonary agents, as they primarily cause damage to the respiratory system upon inhalation. Thus, while cyanide's specific impact on blood oxygen utilization designates it as a blood agent, the others belong to entirely distinct categories of chemical warfare agents.