Review of the U.S. Army's Health Risk Assessments for Oral Exposure to Six Chemical-Warfare Agents (1999) / Chapter Skim
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4: Evaluation of the Army
4: Evaluation of the Army
Pages 36-47
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The Army Surgeon General accepted ORNE's proposed RfD as an interim exposure value until an independent evaluation of the proposed RfD was conducted by the National Research Council (NRC)
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The RfD for GB was calculated to be 2 x 10~ mg/kg per day by dividing the L°AELa<,j by 2,700, the product of the uncertainty factors and the modifying factor selected by ORNL. APPROPRIATENESS OF THE CRITICAL STUDY The critical study used by ORNE for deriving the RfD for GB was a subchronic toxicity study (Bucci and Parker 1992)
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ORNL reanalyzed the data using ANOVA and Dunnett's comparison and reported significantly lower RBC-AChE values in males in all dose groups compared with baseline values at weeks I, 3, and 7 and for the two highest dose groups at week 13. The values for all dose groups were also significantly lower than control values at weeks I, 3, and 7.
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The subcommittee considered other possible toxicity end points, notably neurotoxicity, associated with GB exposure. Organophosphate compounds like GB may act directly on nerve cell receptors or, by inhibiting neural AChE, interfere with neuromuscular transmission and produce delayed-onset subjunctional muscle damage.
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APPROPRIATENESS OF UNCERTAINTY FACTORS For GB, ORNERY assigned values greater than ~ to five uncertainty factors and a value of ~ to the modifying factor. The product of those factors was 2,700.
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Although it could be argued that a dose of GB that significantly induces ChE inhibition in the absence of toxic effects is indicative of a NOAEL rather than a LOAEL, the subcommittee agrees that a factor of 3 is a conservative choice for UFt. EXTRAPOLATION FROM SUBCHRONIC TO CHRONIC EXPOSURES In the derivation of RfDs for other organophosphate compounds, ORNE noted that EPA (1989, 1992)
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is appropriate. MODIFYING FACTOR FOR ADDITIONAL UNCERTAINTY The subcommittee considers the uncertainties of the data on GB to be represented adequately by the values assigned to the uncertainty factors above and agrees with ORNE that a modifying factor (MF)
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On the basis of available toxicity and related data on GB, the subcommittee concludes that the Army's interim RfD for GB of 2 x 10-5 mg/kg per day is scientifically valid.
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At least one dose between O and 0.075 mg/kg per day should be used. If further research reveals that significant toxic effects can be induced by any of the nerve agents evaluated (i.e., GA, GB, GD, or VX)
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Final Report. Prepared by the National Center for Toxicological Research, U.S.
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U.S. Environmental Protection Agency, Health Effects Division, Office of Pesticide Programs, Washington, D.C.
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1998a. Chronic neurobehavioral effects of Tokyo subway sarin poisoning in relation to posttraumatic stress disorder.
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