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...