There are several categories of chemical weapon. Chlorine, for example, is a choking agent, that makes breathing difficult. Cyanide is a "blood agent," not because it enters through the bloodstream but because it makes it harder for blood to transport oxygen. A blistering agent, like mustard, burns skin and causes ruptures wherever it makes contact, so it's especially harmful to eyes and the respiratory tract, and can be fatal—especially if it gets in the lungs. Nerve agents, like Sarin, instead directly attack the nervous system, and work almost immediately; they lead to paralysis or convulsions and, if the respiratory system is attacked, an inability to breath and death shortly thereafter.