Chemical agents are chemical compounds which are designed to cause uncomfortable or fatal symptoms in humans. There are a number of different types of chemical agents which work in different ways, from chemicals designed to be used in lethal chemical warfare to crowd control agents which temporarily incapacitate. Access to such chemicals is tightly restricted, due to concerns about what could happen if the chemicals got into the wrong hands, and the suspected presence of chemical agents in the stockpiles of an enemy nation has been used as a pretext for international inspection and even war.
Humans have been using chemical agents for a very long time. Accounts of Ancient Greek warfare describe the use of flaming ships filled with noxious substances and sailed into the ranks of the enemy to disable them, for example. As knowledge of chemistry increased in the 19th and 20th centuries, the sophistication of chemical agents also increased. World War One saw the use of several chemical weapons in the trenches, with many soldiers being injured or killed by substances like mustard gas.
After the First World War, several nations recognized the potential threat of chemical agents, realizing that they could be used with lethal consequences in warfare. Chemical agents do not discriminate between friend and enemy, or between combatant and civilian, and there were several instances in the First World War when troops accidentally gassed themselves, or gassed civilian populations. As a result of these concerns, several chemical weapons bans have been passed, with signatories agreeing to refrain from producing or stockpiling chemical agents for military use.
To comply with such bans, most nations use a classification system to identify chemical agents. Chemicals in the first class have no use other than as chemical weapons, and these chemicals are often explicitly banned. In the second class, chemicals which have some applications beyond military use are grouped together, while the third class includes common chemicals with a wide variety of uses, including potential military applications. Companies which produce chemicals in the second and third classes may be required to keep careful tabs on who they sell their chemicals to, and the volume of their sales.
The actions of chemical agents are quite varied. Some attack the pulmonary system, causing people to choke to death, while others raise blisters on the skin and in the lungs if they are inhaled. Chemical agents can also interfere with protein production in the body, assault the nervous system, or temporarily incapacitate people. Many police forces take advantage of short-acting chemical agents which are designed for use in riots and mobs to quell a crowd without causing permanent damage; tear gas is an example of a chemical agent which might be used in such circumstances.