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MILITARY UNIT AND HERBICIDE SPRAYING DATABASES, AND EXPOSURE ASSESSMENT MODEL DEVELOPMENT 20 Approach to Exposure Assessment The Columbia University researchers proposed the EOI as a useful alternative to such traditional toxicology- based measures as blood or adipose-tissue concentrations. It was not meant to be a substitute; neither method is perfect, and both can yield valuable information about potential exposure. The EOI method used in this project was originally developed and published by the researchers during the 1980s (Stellman and Stellman, 1986). Its central feature is the comparison of the geographic location of a potentially-exposed military unit with all known locations of herbicide release. Total exposure opportunity for the unit is the sum of the EOI estimates for all temporally-appropriate15 data in the database. Four models to quantitatively assess exposure opportunity were developed by the Columbia University researchers in the course of this and previous work. They incorporate increasingly realistic (and more complex) exposure concepts of distance and time of potential exposure to herbicide application. The simplest, E1 (the âhitâ model), simply counts the instances in which a person was within a specified distance of a known spray. The second, E2, also counts hits but makes close hits count more by weighting each hit according to inverse distance from the spray. The third model, E3, begins with distance-weighted hits and factors in the total time during which the person is considered to have been exposed. E1 and E2 can be regarded as representing acute or direct exposures, since no allowance is made for exposure engendered by entering a sprayed area after the spraying has occurred or for the length of time spent in the sprayed area. E3 is analogous to acute followed by chronic exposure. Time is an essential characteristic of the current, E4 EOI model. Any person or entity that is present on the day of spray would be considered to have âdirect exposureâ. Those entering a sprayed location after that time and those remaining in the location after having been directly exposed would be considered to have âindirect exposureâ, that is, exposure to any residual 15 The exposure assessment software that implements the model allows the user to set the time period over which potential exposure should be factored. This may be relatively short if the user is examining herbicide ingredients that break down in sunlight or extremely long for a chemically-stable compound like dioxin.
