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.)