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REVIEW OF ACUTE HUMAN-TOXICITY ESTIMATES FOR VX 54 ECt50, for Severe Effects CDEPAT's proposed ECt50 estimate for severe effects is 10 mg-min/m 3, assuming exposure durations of 2 to 10 min, minute volumes of 15 liters, and moderate temperatures. The existing ECt50 estimate for severe effects is 25 mg- min/m3. In one study, the ICt50 (exposure level producing incapacitation in 50% of a given population) for VX was estimated to be 13 mg-min/m 3 (Howd et al. 1986). That estimate was based on ChE inhibition in humans and animals. The available data to derive an ECt50 for severe effects are insufficient. CDEPAT used the ratio of ICt50/LCt50 of 0.7 to 0.8 to derive the ECt50 for severe effects. The rationale for using this ratio is unclear. The subcommittee's degree of confidence in the CDEPAT's proposed ECt50 estimate of 10 mg-min/m3 is low. Thus, the subcommittee recommends that the proposed ECt50 estimate for severe effects of 10 mg-min/m3 serve as an interim value until further research on VX is conducted to establish the estimate with a greater degree of confidence. Ect50 for Mild Effects CDEPAT's proposed Ect50 estimate for ocular effects or rhinorrhea in humans is 0.09 mg-min/m3, assuming exposure durations of 2 to 10 min and moderate temperatures. The existing estimate is the same (CDEPAT 1994). Exposures of the head and neck of humans to VX vapor for 1.5 to 6 min resulted in Ect50 values of 0.6 to 6.4 mg-min/m3, respectively (Bramwell et al. 1963). Transient respiratory signs occurred after exposure, but no systemic effects were observed. A vapor dose of 5.5 mg-min/m3 produced a 70% ChE inhibition. Some miosis occurred after exposure. Rhinorrhea was reported in all but one subject. The effects of wind speed on the miotic potencies of VX in humans and rabbits were also studied. A definite relationship was observed between impaction velocity on the cornea (exposure that reduces pupil size by 90%) and the Ect90. On the basis of the available data, the subcommittee concludes that the Ect50 estimate of 0.09 mg-min/m3 for ocular effects is scientifically valid.