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From Citizendium, the Citizens' Compendium
Chemical terrorism is the form of terrorism that uses the toxic effects of chemicals to kill, injure, or otherwise adversely affect the interests of its targets.
While there may be controversy about the definition of the politically-charged word "terrorism," the tactics and technology of chemical terrorism are clearly distinguished from those of other forms of chemical warfare, using chemical weapons designed to meet military needs. Chemical terrorism is asymmetric warfare as practiced by non-uniformed forces using light and/or improvised weapons against non-combatant targets. It is therefore unlike the symmetric chemical warfare of the First World War, in which dug-in troops fired poison-filled artillery shells at each other across a wire-bounded no-man's-land. It is also distinct from asymmetric "terror from above" in which military forces use munitions with chemical payloads against civilian populations.
Chemical terrorism is also qualitatively different from biological terrorism involving infectious diseases, but quite similar to the covert employment of biologically-produced toxins, which differ from synthetic poisons mainly in their extreme potency and the means by which they are produced.
There have been few documented acts of chemical terrorism, and none of those has caused casualties justifying the treatment of chemical weapons as Weapons of Mass Destruction. However, there has been much discussion and some serious study of the possibility of chemical terrorism. One of the stated concerns leading to the 2003 invasion of Iraq was the possibility that chemical weapons technology developed and used by Iraq could be transferred to terrorist organizations.
The main issue in chemical warfare, for high-tech state-funded military users as well as for non-traditional forces, is distributing the material efficiently in the target area. In most chemical warfare scenarios, much or most of the toxic agent will be destroyed by explosive dispersal devices, delivered in massive overkill quantities to a few victims, and/or broadcast into areas where no potential victims exist. If these dispersal devices can be discovered, explosive ordnance disposal teams may render safe the explosive system, or, if that is impossible, conduct a controlled detonation that limits spread of the toxic materials. Toxic agents that do not find victims immediately on delivery may degrade spontaneously, or be deactivated or sequestered by decontamination teams.
No known chemical weapons qualify as "weapons of mass destruction" in the sense of even the Hiroshima and Nagasaki bombs. Realistic chemical attacks will be on a smaller scale, but a campaign of such attacks could be extremely disruptive. The insidious and somewhat mysterious nature of poisons makes them potential weapons of mass terror, because people in a target area - or simply in what they perceive to be a target area - will not know whether or not they've been poisoned.
Methods used by terrorists or hypothesized by analysts include:
- Contamination of reservoirs and urban water supply systems.
- Contamination of food, beverages, drugs, or cosmetics in manufacturing or distribution processes.
- Contamination of food or beverages near the point of consumption.
- Miscellaneous product contaminations: stamps/envelopes, IV fluids, etc.
- Release of gases or aerosols into building HVAC systems.
- Release of gases or aerosols from aircraft.
- Dispersal in bombs or projectiles.
- Miscellaneous direct methods: hand sprayers, water guns, parcels.
- Release of industrial/agricultural chemicals via attacks on production or storage facilities.
- Release of industrial/agricultural chemicals via attacks on truck, rail, or barge shipping.
- Miscellaneous releases of industrial/agricultural chemicals, especially anhydrous ammonia, fumigants and pesticides, and disinfectant gases (e.g., chlorine, chlorine dioxide, ethylene oxide).
The selection of chemical agents
Chemical terrorism could be carried out either with military chemical weapons, or toxic industrial chemicals.
Military "nerve gases" are selected for their extreme toxicity, and are produced in quantity only for use as weapons. The LD50s of these compounds (lethal dose for 50% of exposed humans) are expressed in micrograms of poison per kilogram of victim body weight, making them more toxic by orders of magnitude than even the most dangerous industrial materials. The compounds Tabun and Sarin were developed - but never used - by Nazi Germany, and have since been manufactured and stockpiled by the United States, the Soviet Union, and numerous other countries. More potent compounds, most notably VX and the other V-series compounds, have been developed and weaponized more recently. These well-known agents, as well as their precursors, are listed in the Schedules of the Chemical Weapons Convention, and both administrative security and environmental monitoring measures help nonproliferation.
The choice of toxic agents for use in the improvised weapons of asymmetric warfare is constrained by the availability of usable poisons. This is obviously true in the selection of sabotage targets from which existing stocks of hazardous materials can be released, but it is also true in general: without a state sponsor, or the ability to steal from military arsenals, or the sort of heroic efforts made by the Aum Shinrikyo cult to synthesize its own Sarin, military-style supertoxic weapons must be considered unavailable to non-state organizations.
Some pesticides share characteristics with military chemical agents. Tabun was originally developed in a German search for new pesticides, and Amiton, the earliest V-series agent, was actually brought to market as a pesticide by a British company. Manufacturing technology and raw materials that can be used to make pesticides may also be used to make chemical warfare agents. The most dangerous pesticides have been largely or completely replaced by more selective alternatives that kill pests effectively with less danger to humans. Several highly-toxic "restricted-use" pesticides are still produced and used in very large quantities, and could be hijacked by the truckload. This includes parathion, methyl parathion, and other organophosphorus compounds with LD50s on the order of 10 mg/kg. Military chemical sensors test for such materials, as well as specific chemical weapons.
As alternatives to highly-toxic pesticides become available, the older and more dangerous substances are de-certified for commercial use, and are either dropped entirely or made only for export by the chemical industry. However, some of these are relatively simple compounds that could be made in clandestine labs. For example, TEPP, the first and most dangerous organophosphorus pesticide, though significantly less toxic than Tabun, Sarin, and VX, is nevertheless fast-acting and deadly enough for use in direct attacks on soft targets, and its relatively simple synthesis is described in old patents.
It is to be expected that certain rodenticides would be extremely effective as contaminants, since their normal application requires them to be stable, odorless, and tasteless while possessing high mammalian toxicity. Modern rat and mouse killers meet these criteria without creating extreme hazards for humans. Arsenic, on the other hand, is the classic example of a rat poison that is equally applicable to homicide. Inorganic thallium, barium, and phosphorus compounds might also be used, although some of these will require high concentrations for reliable lethality.
Much more potent than arsenic are two widely-banned rodenticides: sodium fluoroacetate, also known as Compound 1080, and tetramethylenedisulfotetramine, sometimes called TETS or "tetramine." The human LD50 for these substances is on the order of 1 mg/kg, with TETS being perhaps 3-10 times stronger than 1080. No antidote is known for either agent. TETS has been widely discussed as a potential terrorist weapon, having been used in several multi-fatality crimes of private revenge in China, where there exists a black market for illegal TETS-based rat poisons made in secret factories. Very small amounts of Compound 1080 are used legally for predator control in the United States, but several tons per year are made for export by Tull Chemical in Alabama. Because the use of 1080 to kill predators has been very controversial, environmental organizations have stressed its potential as a terrorist weapon in their attempts to have it outlawed completetely.
The most common toxic hazardous materials are chlorine and anhydrous ammonia. While chlorine is normally stored and shipped in very large containers, the use of ammonia in agriculture requires it to be distributed to many more sites in smaller containers. These gases create possibilities for highly disruptive large-scale releases, but will cause few or no fatalities unless victims are trapped in areas where concentrations are high. Recent suicide bombings in Iraq have combined conventional explosives with chlorine tanks; while this technique hasn't produced exceptionally high death tolls, it causes increased chaos and can be expected to drive all unprotected survivors away from the scene of an attack.
There are efforts to reduce the availability of chlorine gas. In water purification, chloramines are being used as an alternative. Clorox, the largest bleach manufacturer in the U.S., has changed its manufacturing process so that it no longer uses elemental chlorine. 
Part of the severity of the Bhopal Disaster was that large quantities of methyl isocyanate were stored, and in a plant not operating. A safer approach is to make toxic feedstocks as they are needed, storing as little as possible.
The Aum Shinrikyo scenario
On the morning of March 20, 1995, the Tokyo subway system was hit by synchronized chemical attacks on five trains. Using simple lunch-box-sized dispensers to release a mixture containing the military nerve agent Sarin, members of the Aum Shinrikyo religious cult killed twelve people and injured about a thousand others. The incident was unusual because the cult was using nerve gas that it had made in its own facilities; however, using unsophisticated means to disperse this low-quality agent, the attackers produced results less impressive than those achieved with ordinary explosives in the attacks on the 2004 Madrid and 2004 London bombings.
Because Aum Shinrikyo had apparently spent millions of dollars to develop its chemical warfare capabilities - many times more money than was spent to arm the Madrid and London bombers - its example will probably not be followed by many other groups. However, the public and media fascination with chemical "weapons of mass destruction" tends to focus on the large numbers of lethal doses that can theoretically be derived from a small amount of material, and to neglect the practical problems of distributing those doses to the intended victims. An example of this was the publicity given reports of an alleged al Qaeda plan to use "mubtakkar" cyanide generators in the New York subway system. The reporters covering the story apparently didn't understand that no technical breakthrough was necessary for making a compact cyanide generator, or that the Tokyo incident had already shown that subway trains, with their doors and ventilation systems, are poorly-suited to use as gas chambers.
The Bhopal scenario
Chemical warfare as we know it today could not have existed prior to the emergence of a large-scale chemical industry in the nineteenth and twentieth centuries. The industry's need for very large quantities of highly reactive intermediate compounds put it in the business of making, storing, and safely using thousands of tons of extremely dangerous substances. Early war gases like chlorine, phosgene, and hydrogen cyanide were economically-important products of the chemical industry before they were selected for use as weapons. The routine use of so much dangerous material by industry could provide the leverage to turn acts of sabotage into major disasters.
While not an act of terrorism, the obvious model for such an attack is the December 1984 disaster at a Union Carbide pesticide plant in Bhopal, India. Thousands of Bhopal residents were killed by the accidental late-night release of about thirty tons of methyl isocyanate into the atmosphere. Hundreds of thousands of victims suffered permanent injuries, from which many have died in later years. However, this is an extreme worst-case scenario - probably the worst industrial disaster in history - and would be very difficult to recreate in more developed countries.
The very high population density around the plant, while not unusual for Indian cities, would not be typical of the neighborhoods in which comparably dangerous facilities would be sited in the United States or Europe. The plant's safety systems had fallen into a degree of disrepair that would be considered intolerable in most cases.
Although the possibilities for large-scale chemical accidents obviously creates a need for public information about hazards concealed within privately owned facilities, this information is frequently suppressed, either because the proprietors of dangerous facilities prefer to avoid public notice, or because authorities hope to keep that information out of the hands of terrorists. However, any serious attempt at this kind of terrorism would involve a detailed study of potential targets by people with a good understanding of chemical engineering, making the removal of hazard information from the public domain naive at best.
The Nanjing snack shop scenario
In September 2002, hundreds of people were made ill and several dozen died after eating breakfast at a snack shop in Nanjing, China. The owner of a competing restaurant had poisoned the shop's products with a TETS-based rat poison, apparently because of jealousy of his rival's success.
In this case, because the attack was close to the consumers of the contaminated product, there was no need to obtain, store, transport, or mix the large quantities of poison that would be needed for attacks on bulk food supplies. One can easily imagine a single employee at a fast-food restaurant or special-event concession stand contaminating a much-used item like salt, cooking oil, or the soy milk that may have been the carrier in the Nanjing attack. Using a slower-acting poison would make identification of the source of the poison more difficult. Staging a set of simultaneous attacks would increase the number of victims, and would greatly increase panic because of uncertainty about the number and location of attacks.
Because of the great number and high turnover of low-skill low-wage employees in the fast-food industry, there is no way to rule out the possibility of such an attack, which would be more likely to be defeated by luck and the attentiveness of a co-worker or manager than by anything security agencies might do.
The WMD scenario
How might terrorists make use of a military chemical weapon? As discussed above, the toxicity of military nerve agents would far exceed that of any compound that would be available from ordinary civilian sources. However, sophisticated chemical munitions would be of little value without the complete weapon systems for which they're designed.
Some munitions are designed to function as line sources, releasing the agent into the air during a segment of the flight of a bomb or shell. Simpler munitions are point sources, releasing their entire payload at the single point where the bomb, shell, or rocket bursts. A line-source weapon, if designed and used effectively, provides a means to broadcast the agent over a larger area, bringing it into contact with more people. However, such a weapon will have been designed to operate at a certain airspeed and altitude, obeying commands from a certain control system. The terrorist, lacking the appropriate howitzer or smart bomb system, will either have to take on the very dangerous task of removing the agent from the munition to repurpose it, or use the munition in an improvised point source weapon by blowing it up with other explosives.
For safer handling and storage, many modern chemical weapons are binary munitions, which contain not an active toxic agent but two compounds that must be mixed to form the agent. If such a weapon is not used as designed, it is unlikely to be effective. A simple example of such a munition is an artillery shell in which the separation between the two chemical components is broken when the shell is fired. The two compounds mix as the round spins in flight, and an explosive charge bursts the shell only after the reaction to form the active compound is complete. Using such a round in a improvised explosive device would simply destroy it.
Fortunately, grim fantasies of terrorists using a military chemical weapon to take out an entire city have little basis in reality. Solutions to the difficult problems of handling chemical agents safely and dispersing them efficiently are too complex to be transplanted from military weapons systems to improvised devices.
Hypes and hoaxes
The questionable idea that poison can be "the poor man's nuclear weapon" has been amplified by wild claims that exaggerate both the lethality of such weapons and the ease of making them. Underground American publications like "The Poor Man's James Bond" have been taken too seriously by would-be jihadists and/or by people in the news media and law enforcement who want to make frightening statements about terrorist threats. The mundane origins of powerful biological toxins have been exploited to create stories about how thousands of people might be killed by potions concocted from cigarettes, castor-oil beans, sprouting potatoes, rotten meat, or human excrement.
Ricin, a very powerful toxin obtained from the castor-oil plant, has been involved in some of these stories as well as in actual prosecutions. Allegations that a British al-Qaeda cell had been plotting mass murder by ricin were cited by Colin Powell in his presentation to the U.N. Security Council as evidence of the Saddam Hussein regime's involvement in terrorism. The actual evidence for the "plot" was little more than a useless ricin recipe downloaded from the Internet. All suspects were eventually freed except for one who had actually killed a detective (with a kitchen knife).
A reasonably open society with a well-developed chemical industry provides many vulnerabilities that might be exploited by skilled, committed, and adequately-funded makers of improvised chemical weapons. However, the logistical and engineering challenges presented by such mass poisoning scenarios as those described above will not be overcome by mere intent.