Chemical Terrorism

By James A. Geiling, MD, FCCP

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Agent Weaponization and Physical Characteristics

Although they are relatively easy to make in the laboratory, use of many of the known biological and chemical agents as terrorists' weapons can be difficult. Many factors affect the dispersion and effectiveness of the agents. Agents may require a narrow range of atmospheric or meteorologic conditions in order to remain physiologically active, such as the requirement for an inversion to maintain a chemical agent's persistence. Additionally, weaponization of the agent can induce other challenges, such as hot gases from an explosion that limit the agent's effectiveness. Some agents work directly against their target, whereas others, particularly bioweapons, may be more effective using a vector for dissemination. Some agents are best released at a point source, whereas other may require line dissemination as from a vehicle such as an airplane. Finally, residue effects from persistent agents can affect how terrorists use the agents and the overall time course of effects.

Chemical agents can be divided into broad classifications based on their mechanisms of action and physiologic effects. The most common categories include: (1) pulmonary agents; (2) cyanide; (3) vesicants; (4) nerve agents; (5) incapacitating agents; and (6) riot control agents.

The pulmonary agents include phosgene (CG) [carbonyl chloride] and diphosgene (DP) [trichloromethyl chloroformate]. A review of their characteristics can be found in Table 1. Hydrogen cyanide (AC) and cyanogen chloride (CK) have in the past been known as "blood agents." A summary of these agents is found in Table 2. The most common vesicant used in warfare has been mustard (HD) [bis(2-chloroethyl)sulfide] (Table 3). Other vesicants available include lewisite (L) [2-chlorovinyldichloroarsine] (Table 4) or a mustard-lewisite mixture (HL).

The agents with the greatest reputation for terror are the nerve agents. These include the volatile agents tabun (GA) [ethyl N,N-dimethyl-phosphoramidocyanidate], sarin (GB) [isopropyl methylphosphonofluoridate], soman (GD) [1,2,2-trimethylpropyl methylphosphonofluoridate], and GF (cyclohexyl-methylphosphonofluoridate). VX, or O-ethyl-S-[2-(diisopropylamino)ethyl] methylphosphonothiolate, is a persistent nerve agent of low volatility. Summary information on these agents is in Table 5.

Finally, both terrorists and law-abiding government agencies can use nonlethal agents that have been developed. They include the incapacitating agents BZ (QNB) [3-quinuclidinyl benzilate] (Table 6) and riot-control agents or tear gas, of which there are two versions–CN [2-chloro-1-phenylethanone], or Mace (Mace Security International, Inc; Bennington, VT), and CS [2-chlorobenzalmalononitrile] (Table 7).

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