Chemical weapons are weapons that achieve their deadly effects not by explosion, heat, or penetration, but by biochemical interaction with the target. Chemical weapons include agents that act on the nervous system, such as sarin and VX, and those that affect all exposed tissues, such as mustard gas. Other chemical weapons include botulinum toxin, hydrogen cyanide, psychoactive compounds such as phencyclidine, and tear gas. The use of chemical weapons in warfare has been regulated by a series of international treaties, although some nations have not signed these agreements and continue to use them.
The most common chemical weapons are those that act on the nervous system. These are often called nerve gases, but in fact very few chemical weapons are actually gases. Most nerve agents are organophosphorus compounds, including sarin (isopropyl methylphosphonofluoridate), the first nerve agent, which was widely produced beginning in the 1930s. Since then, it has been supplanted in most modern arsenals by VX (O-ethyl S-diisopropylaminomethyl methylphosphonothiolate), which persists longer in the environment. Another organophosphate, tabun (O-ethyl dimethylamidophosphorylcyanide), is the easiest to manufacture, and is therefore more likely to be manufactured by less developed countries or by small terrorist organizations. These compounds can be prepared as binary agents, in which the final product is formed from two precursors just before release, or during delivery of the shell or warhead. This increases the safety of handling of the weapon.
All organophosphates act by binding to and inactivating the enzyme acetylcholinesterase. This enzyme normally breaks down the neurotransmitter acetylcholine, which stimulates muscle contraction, as well as performing other functions within the body. Without acetylcholinesterase, muscle cells and other organs become overactive. Weakness, tremors, and convulsions follow exposure. Respiratory paralysis is the direct cause of death. Organophosphates are also used as insecticides, and rely on the same mechanisms of action for this application.
Mustard gas (bis-[2-chloroethyl]sulphide), more properly called mustard agent, was first used as a chemical weapon in World War I, where it was responsible for the deaths of large numbers of soldiers on both sides. It was also used by Iraq during the Iran-Iraq war of 1979-1988, where it is though to be responsible for up to 1,000 deaths, and many more injuries. Mustard agent acts by binding covalently to many different types of molecules, disrupting their structures and destroying their normal functions. Damage is delayed for a number of hours after exposure, making it a less effective weapon in this respect than nerve agents.
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