1. ^aM.E. Ginevan & Associates, 307 Hamilton Avenue, Silver Spring, Maryland 20901, USA
2. ^bInfoscientific.com Inc., 5233 Marimoore Way, Carmichael, California 95608, USA

Correspondence: Dr Michael E. Ginevan, M.E. Ginevan & Associates, 307 Hamilton Avenue, Silver Spring, MD 20901, USA. Tel.: +301 585 4951; Fax: +301 585 1350; E-mail: michael@ginevan.com

Received 10 August 2007; Accepted 7 December 2007; Published online 12 March 2008.

Abstract

The AgDRIFT aerial dispersion model is well validated and closely related to the AGDISP model developed by the USDA Forest Service to determine on- and off-target deposition and penetration of aerially applied pesticide through foliage of trees. The Exposure Opportunity Index (EOI) model was developed to estimate relative exposure of ground troops in Vietnam to aerially applied herbicides. We compared the output of the two models to determine whether their predictions were in substantial agreement, but found a total lack of concordance. While the AgDRIFT model estimated that ground-level deposition through foliage was reduced more than 20 orders of magnitude at less than 1 km from the flight line, the EOI model predicted deposition declines less than one order of magnitude 4 km from the flight line. Interestingly the EOI model predicts a four-fold variability in EOI on the flight line, where exposure should be essentially invariant because the spray apparatus is designed to apply herbicide at a constant rate. We believe that the EOI model cannot be used to provide individual exposure estimates for the purpose of conducting epidemiologic studies. Moreover, evaluation of the position data for both herbicide spray swaths and troop locations, together with the actual patterns of spray deposition predicted by the AgDRIFT model, suggests that precise individual-level exposure assessments for ground troops in Vietnam are impossible. However, we suggest that well-validated tools like AgDRIFT can be used to estimate exposure to groups of individuals.