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REVIEW OF ACUTE HUMAN-TOXICITY ESTIMATES FOR VX 51 The potency of VX is enhanced by increased ambient wind speed and aerosol particle size. Those factors are more noticeable with VX than with the other nerve agents because of the relatively low volatility of VX. The recommended percutaneous estimates for VX vapor are for no wind and for aerosol particles of < 2 Âµm diameter (CDEPAT 1994). The estimates also take into consideration the areas of the body that are probably the most sensitive to VX, the head and neck, which are highly likely to be exposed to vapor (Bramwell et al. 1963; Sim 1962). The most relevant human and animal studies for generating human LCt 50 estimates are discussed below. The percutaneous toxicity of VX vapor was investigated in mice and goats. The exposure duration was uncertain for goats (values were given, but uncertainty was associated with them) and unreported for mice (Koon et al. 1960). The LCt50 was 11.5 mg-min/m3 for mice and 100 to 150 mg-min/m3 for clipped goats (Koon et al. 1960). The LCt50s for clothed, depilated rabbits at 0-mph and 8-mph wind speeds were 814 mg-min/m3 and 35 mg-min/m3, respectively (Cresthull et al. 1963). The values for unclothed rabbits at 0-mph and 8-mph wind speeds were 28 mg-min/m3 and 8.3 mg-min/m3, respectively (Cresthull et al. 1963). Krackow (1956) studied the toxicity of VX in depilated rabbits. On the basis of his data, he proposed an LCt50 of 124 to 180 mg-min/m3. Animal data indicate that the highest observed LCt50 for no wind is 100 to 150 mg-min/m3 (Koon et al. 1960). Human estimates vary from 6 to 3,600 mg- min/m3 for various exposure conditions (Koon et al. 1960), and the human estimate depends on wind speed and particle size. Higher wind speed and larger particle size are more effective in producing toxicity of VX. The more credible animal studies estimated the LCt 50 to be between 280 and 300 mg-min/m3. CDEPAT estimated a LCt50 for percutaneous VX vapor of 150 mg-min/m3 (slightly more protective given the sensitivity of the head region). The subcommittee agrees with CDEPAT's evaluation that the degree of confidence in that estimate is low. The subcommittee recommends that CDEPAT's proposed LCt50 estimate be considered an interim value until further research on VX is conducted to establish the LCt50 estimate with a greater degree of confidence. ECt50 for Severe Effects CDEPAT's proposed ECt50 estimate for severe effects from percutaneous