esterase properties. These compounds were developed as chemical warfare agents, and one (agent GB, or sarin) was used by terrorists in the 1995 exposure incident that took place in the Tokyo subway system. The chemical names of these five agents are as follow: agent GA, dimethylamidocyanoethylphosphate (CAS Registry No. 77–81–6); agent GB, isopropyl methylphosphonofluoridate (CAS Registry No. 107–44–8); agent GD, pinacolyl methylphosphonofluoridate (CAS Registry No. 96–64–0); agent GF, O-cyclohexylmethyl-fluorophosphonate (CAS Registry No. 329–99–7); and agent VX, O-ethyl-S-(diisopropylaminoethyl) methyl phosphonothiolate (CAS Registry No. 50782–69–9).

The G agents are all viscous liquids of varying volatility (vapor density relative to air between 4.86 and 6.33) with faint odors (“faintly fruit,” or “spicy,” odor of camphor). Toxic effects may occur at vapor concentrations below those of odor detection. Agent VX is a amber-colored liquid with a vapor density of 9.2 (air=1) and is considered odorless. As a consequence, agent VX vapor possesses no olfactory warning properties.

The vapor pressures and acute toxicity of these agents are sufficiently high for the vapors to be rapidly lethal. Within the G-series, GB is considered a greater vapor hazard than agent GD. Agent GA represents a smaller vapor hazard and is expected to present a relevant contact hazard. The vapor density of agent GF is intermediate between that of agents GA and GD. Agent VX, which has a vapor density (9.2) greater that of any G agent under consideration, was deliberately formulated to possess a low volatility; VX is approximately 2,000 times less volatile than nerve agent GB (DA 1990). As a consequence, agent VX is a persistent, “terrain denial” military compound with the potential to off-gas toxic vapor for days following surface application.

Exposure to acutely toxic concentrations of nerve agents can result in excessive bronchial, salivary, ocular, and intestinal secretions and sweating, miosis, bronchospasm, intestinal hypermotility, bradycardia, muscle fasciculations, twitching, weakness, paralysis, loss of consciousness, convulsions, depression of the central respiratory drive, and death. Minimal effects observed at low vapor concentrations include miosis (contraction of the pupils of the eye, with subsequent decrease in pupil area), tightness of the chest, rhinorrhea, and dyspnea (Dunn and Sidell 1989).

The results of agent GB vapor exposure studies conducted with human volunteers indicate that the threshold for miosis and other minimal toxic effects falls in the range of 0.05–0.5 mg/m3 for 10–30 minute (min) exposures. The findings are based on the results of low-concentration nerve agent exposures of informed volunteers who were under clinical supervi-

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