Definitions

Classic Definitions of Chemical and Biological Warfare Agents

The accepted classic definition of a chemical warfare agent is a chemical substance that is intended for use in military operations to kill, seriously injure, or otherwise incapacitate humans through pathophysiologic effects.^[17] Traditionally, riot-control agents and herbicides were excluded from this definition, although they have frequently been used in a way that would be compatible with the definition of a chemical warfare agent. Biological warfare agents were defined by the 1972 Biological Warfare Treaty as living organisms, whatever their nature, or the infective materials derived from them that are intended to cause disease or death in humans, animals, or plants.^[16] Toxins, which are essential parts of the pathophysiologic

2499

The Chemical-Biological Hazard Spectrum

Although CBW agents have traditionally been considered separately, it is appropriate medically to regard them as part of a continuous spectrum of hazards. ^[18] This is shown diagramatically in Figure 64-1 . Agents are arranged in ascending order of molecular weight from chemical toxic agents to self-replicating agents such as bacteria and viruses. The spectral approach to hazards is useful in emphasizing that agents from different parts of the spectrum act in a similar way on the body. The failure of the neuromuscular junction in response to nerve agent anticholinesterases and botulinum toxin is a good example. Bacteria exert their poisonous effects through toxins that may affect a variety of somatic systems. The advantage of the spectral approach is that it serves as a reminder that medical management of CBW injury should respond primarily to system dysfunction rather than to specific etiologic factors.

Essential Properties of Chemical and Biological Warfare Agents

There are four essential properties of hazards within the CBW spectrum: toxicity, latency, persistency, and transmissibility.^[9] These four characteristics are common to chemical and biological agents and determine the degree of risk and the appropriate response. Toxicity is a familiar concept in anesthesia, and toxic effects appear with a specific latency, which is important for the use of agents in deliberate release. In general, chemical agents and toxins have short periods of latency before specific signs and symptoms appear. In contrast, classic biological warfare agents have extended latency periods (usually familiar as incubation periods) before the effects of the induced disease begin to appear. Persistency refers to the ability of a toxic agent to remain in the environment into which it had been released and is a function of the physicochemical properties of the agent. For chemical agents, the persistency may be variable, but for most biological warfare agents, with the exception of spore-forming agents such as anthrax, persistency is usually very limited. Transmissibility may take place as a result of the physical contamination of the victim due to a persistent chemical agent or as a result of infection in the case of an airborne agent. Transmissibility

Figure 64-1 The chemical-biological hazard spectrum. Agents originally classified as chemical or biologic are arranged in order of ascending molecular weight. Toxins, originally classed as biologic agents, occupy the middle of the spectrum, together with neuropeptides, which normally act as transmitters in the central nervous system. These agents, sometimes called agents of biologic origin (ABOs), can be synthesized using genetic engineering techniques. (Adapted from Baker DJ: Anesthesia in extreme environmental conditions. Part 2. Chemical and biological warfare. In Grande CG [ed]: Textbook of Trauma Anesthesia and Critical Care. Baltimore, Mosby-Year Book, 1993, p 1331.)

In summary, toxicity and latency determine the management of the casualty, whereas persistency and transmissibility determine the management of an incident involving the release of a CBW agent.

Toxicity and Its Modifications

Toxicity is usually expressed in terms of LD₅₀ or LCt₅₀ , in which C is the concentration of agent inhaled for time (t) required to produce lethality (L) in 50% of the exposed population. LD₅₀ usually relates to toxicity by means of injected routes. For most chemical agents, the inhaled route is normal, and therefore expressions of concentration and time are used. Haber defined a lethality coefficient as follows:

W = C × t

In this equation, C is the inhaled concentration of toxic agent, and t is the time of exposure. In practice, the absorbed amount of agent depends on the respiratory minute volume of the exposed person. This is just one factor modifying the expression of toxicity. Others include life-support responses in the case of respiratory failure and the effects of antidotes. Specialized texts^[7] provide a more detailed discussion of these factors.

Nuclear, Biological, and Chemical Weapons: The NBC Classification

Since the end of World War II, nuclear, biological, and chemical weapons have by convention been classified as NBC agents. The origins of this system are obscure, but the classification gave rise to a whole subsection of warfare that was developed by both sides during the Cold War. Although all three weapon systems have toxic elements, which necessitated the development of detection, protection, and decontamination procedures, the classification ignored the very intense level of physical damage caused by nuclear explosions. Over the years, the term weapons of mass destruction has been applied to NBC agents, although the term was originally coined by the United Kingdom in a review of nuclear and biological weapons produced at the end of World War II.^[13] The term NBC agents has been altered to reflect the threat perception from the use of explosive devices that can spread radioisotopes

2500