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MILITARY UNIT AND HERBICIDE SPRAYING DATABASES, AND EXPOSURE ASSESSMENT MODEL DEVELOPMENT 21 herbicide or dioxin that is present. The E4 EOI may be expressed mathematically as a product of three factors: ⢠Concentration of the toxicologically active substance in the herbicide mixture. ⢠A person's distance from spray paths. ⢠Residence time at an exposed location. Thus, E4 EOI=concentration factorÃdistance factorÃtime factor. The E4 EOI for a given mission is calculated as the sum of the component E4 values for all its legs. The researchers expanded the scope of assessment for the models in three important ways: by giving greater consideration to the questions of dispersion of individual troops than to reported locations of their units, using nonlinear error methods; by carrying out sensitivity analyses to account for spray-coordinate errors introduced by deviation of aircraft from flight paths and wind dispersion of herbicides; and by investigating other exposure models as extensions of or alternatives to the above. Two elements needed improvement for exposure-opportunity models: incorporation of gallonage of herbicide and consideration of connectivity of fixed-wing aircraft flight paths. The intent of this work was thus to attempt to improve the modeling of herbicide dispersal by creating a mathematical means of estimating the herbicide release along the entirety of flight paths rather than at points along flight paths. Modeling Herbicide Exposure As the project evolved, the Columbia University researchers refined their approach to calculating the EOI. Rather than estimating it directly for a military unit on the basis of its location, they chose to calculate an exposure score for a series of contiguous grids that cover Vietnam and a military-unit occupancy probability, which represents the likelihood that an individual military unit will occupy any specific grid. These grids are 0.01° Ã0.01°, which is
MILITARY UNIT AND HERBICIDE SPRAYING DATABASES, AND EXPOSURE ASSESSMENT MODEL DEVELOPMENT 22 equal to ~1.2 km2 on average. The gridding concept permits use of statistical methods to evaluate exposure of units whose locations are known only probabilistically. It also makes it possible to superimpose contours of exposure on maps of Vietnam to visualize regions of especially high and low exposure. Finally, it permits more rigorous treatment of the various geophysical characteristics that may affect exposure to and bioavailability of herbicides (soil type, presence of water, and the like). The geographic information system (GIS) developed by the researchers transforms exposure estimation into a more efficient process. The researchers digitized an existing soil map of the former Republic of Vietnam that was incorporated into the GIS to extend exposure modeling to account for differential environmental decay of herbicides (or dioxin) that may depend on soil typology. The map was derived from a 1961 field survey that was carried out by the UN Food and Agricultural Organization (FAO) for the Vietnam Ministry of Agriculture (Moormann, 1961). The original FAO map was scanned, and each of the 925 contours was traced manually. A point-in-polygon algorithm was used to assign to each of the GIS grids an integer corresponding to the soil contour in which it fell. This allows the EOI for a grid to be computed by using a half-life appropriate to the soil type. To make the GIS more easily accessible to researchers, the contractors developed a userfriendly software interface called âHerbicide Exposure Assessment-Vietnamâ (HEA-V). The HEA-V employs information from various components of the databaseâflight paths of aerial spray missions, number of gallons sprayed (gallonage), and chemical agents; documented spraymission targets; herbicide storage, transport, and unplanned-dispersal information; military-unit identification codes; locations of military units, bases, structures, air fields, and landing zones; movements of combat troops; land features16; soil typology; and locations of civilian populationsâto calculate exposure opportunity. A paper published in the March 2003 edition of the journal Environmental Health Perspectives details the development and application of the GIS and associated software system (Stellman SD et al., 2003). 16 Including coordinates of a variety of: elevations and land contours, rivers and streams, mountains and highlands, coastal areas and mangrove forests, bays and estuaries, and such structures as roadways and utilities.
