Chemical weapons fall into four general categories: choking agents, blistering agents, blood agents, and organophosphate nerve agents.
Choking agents, which were extensively employed during World War I, are gasses like chlorine and phosgene. In today's world, these agents really are no alternative to the nerve agents discussed below. In order to be effective, they can only be used in an enclosed space, and it takes a very large amount of the gas to be anything other than annoying.
Similarly, blistering agents, like mustard gas, which was a frequent World War I weapon, do not have a meaningful place in today's world. The one exception would be a circumstance where a terrorist organization wanted to tie up as much of an area's medical facilities as possible but without any widespread lethal results. The consequences of a blister-gas attack are painful and very disruptive but not usually fatal.
Most blood agents are based on cyanide and can be used in lethal amounts. Since lethal cyanide blood levels are similar to lethal levels of phosgene, there really is little difference between blood agents and lethal choking gases. Both require enclosed spaces to be effective, and neither is as effective as nerve agents.
Organophosphates originally were developed as insecticides. In the early part of the twentieth century, there was even limited production and use of early versions of what are now called nerve agents-before their lethal effect on animals was fully understood. Currently, four substances occupy the nerve-agent platform. Their ease of manufacture, their volatility, and their lethality all affect which of these is more suited to terrorist activities.
VX nerve agent is very lethal. Six to ten milligrams of this substance absorbed through the skin or ten milligrams breathed into the lungs as an aerosol will kill. For reference, a grain of rice weighs about ten milligrams. VX is stable; it sticks to things as a slightly sticky, oily film. Its high toxicity and ability to remain on surfaces for a long time make this agent an excellent killing weapon. Fortunately, it is very difficult to manufacture, so VX is unlikely to appear as the next terrorist weapon of choice.
A Soman nerve agent dose is three to five times the size of a lethal VX dose. Soman is more volatile than VX, so it dissipates more quickly; but its main use still is to coat surfaces rather than as a gas. Furthermore, it is just as difficult to manufacture as VX, so given a choice between Soman and VX, a terrorist will probably choose VX.
Both Sarin and Tabun are considerably less lethal, requiring about fifteen times the breathing dose and 200 times the skin dose of VX to be lethal. Sarin is very volatile, so that it is most effective as a gas. This makes it excellent for use in enclosed areas like a large building or a subway. Because of restricted access to intermediate substances required in its manufacture, however, it is difficult to make. Tabun, the oldest nerve agent, while more volatile than VX and Soman, is significantly less volatile than Sarin, and it is very easy to manufacture. It is probable, therefore, that most terrorist nerve-gas SIS operations will use Tabun.
Following World War II, the Allies seized over twelve thousand tons of Tabun. Much of this was subsequently destroyed, but there may still be large quantities stashed in some forgotten warehouse. Given the worldwide presence of terrorist groups such as al Qaeda, it is distinctly possible that World War II vintage Tabun nerve agent forms a significant part of the arsenals of these organizations.