The Utility of Proximity-Based Herbicide Exposure Assessment in Epidemiologic Studies of Vietnam Veterans (2008) / Chapter Skim
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3 Assessment of the Model and Its Capacity to Produce Useful Exposure Metrics
3 Assessment of the Model and Its Capacity to Produce Useful Exposure Metrics
Pages 35-64
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From page 35... ...
software tool. In this chapter, the committee elaborates on the concept of an exposure assessment hierarchy, as introduced in Chapter 2, to serve as the context for its assessment of the Stellman team's model.
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From page 36... ...
Placing the Stellman Team's Model in an Exposure Assessment Hierarchy As noted in Chapter 2, an exposure assessment hierarchy can help illustrate both the relationship between an environmental exposure and a health outcome and the levels at which "exposure" might be measured with greater or lesser accuracy. More specifically, Figure 3-1 illustrates the exposure assessment hierarchy that the committee used to guide its thinking on herbicide spraying in Vietnam and the level at which the Stellman team's model operates.
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Proximity-Based Surrogates of Exposure in Vietnam The exposure assessment hierarchy is instructive in comparing the exposure metrics generated by the Stellman team's model and the exposure assessments used in previous epidemiologic studies of Vietnam veterans.
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From page 38... ...
A few studies have included an effort to assess the location of study participants in relation to recorded herbicide spray locations. More detailed exposure assessments, including serum TCDD measurements, have been made for those military personnel who were part of the Operation Ranch Hand units or the Army Chemical Corps, both of which applied herbicides in Vietnam.
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found no relationship between agricultural herbicide and insecticide concentrations in house dust and self-reported proximity to crop fields in nonfarm households; distance was classified in quarter-mile increments, ranging from less than 0.25 miles to more than 1 mile. INFRASTRUCTURE OF THE STELLMAN TEAM'S MODEL In view of the merit of the Stellman team's proximity-based approach as a reasonable step toward more accurate herbicide exposure assessment, the committee reviewed the components that are the infrastructure of the GIS and the HEA-V software.
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have described assembling this database by cleaning, combining, and reconciling data on spraying from several sources, including records on Operation Ranch Hand missions and U.S. Army helicopter and ground spraying activities (sources known as the HERBS and Services HERBS tapes)
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(The delivery method is not specified for the remaining 630 missions, 70 percent of which were recorded as being for perimeter spraying around base camps, fire bases, air bases, and other fixed military camps.) Records of helicopter missions were kept with Ranch Hand records beginning in 1968, but ground spraying was not tracked as part of a permanent record system (Stanton, 1989)
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From page 42... ...
, the HEA-V data do not incorporate any explicit estimates of TCDD levels for individual spray missions. The available records show that 95 percent of the herbicide used was applied via the missions flown by fixed-wing aircraft as part of Operation Ranch Hand (Stellman et al., 2003a)
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Evaluation of the Infrastructure of the Stellman TEAM'S Model In evaluating the infrastructure of the Stellman team's model, the issues of principal concern to the committee included the general completeness of the data, the completeness of data on herbicide spraying conducted separately from Ranch Hand flights, the potential for errors in the location of spraying arising from errors or imprecision in UTM coordinates or from the representation of flight paths as straight lines, and the appropriateness of assumptions about the extent of the area considered exposed to herbicide by a given mission. The committee included in its considerations limitations of the model noted by the Stellman team (e.g., Stellman and Stellman, 2004)
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. This issue is discussed again later in the chapter, but the committee notes here that considering the wider area when assessing exposure opportunity would seem to address, at least to some extent, the concern that true flight paths may have deviated from the straight lines used in the model.
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If they do, it would be appropriate to consider adding that data to the Stellman team's GIS. THE STELLMAN TEAM'S EXPOSURE OPPORTUNITY METRICS As previously described, the Stellman team's model produces two exposure metrics that are based on proximity to herbicide spraying: hits and the EOI.
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Indirect Exposure Opportunity The EOI calculation for each mission includes three main components: the amount of herbicide sprayed, a GIS cell's distance from the spray path, and an herbicide decay rate (Stellman and Stellman, 2003)
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However, there is support for use of distances much greater than the nominal width of a plane's spray swath for an exposure opportunity metric. A 1969 report on herbicide use in South Vietnam included calculations that, under unfavorable but acceptable operating conditions, spray drift damage to broadleaf crops could occur at distances up to 2 kilometers (Darrow et al., 1969)
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Indirect Exposure Opportunity Concerns have been raised about some of the components of the EOI calculation: the herbicide decay rate, distance from the spray path as an exposure modifier, and the amount of herbicide sprayed. The decay rate is currently modifiable within the user interface of the software; therefore researchers who can support other half-life choices are free to use them for calculating the EOI.
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From page 49... ...
It is possible, however, that drift from ground or helicopter spraying operations would have been less than that for spraying by fixed-wing aircraft, resulting in less exposure at a distance but more exposure near the spray path. Those operations may have been substantial contributors to exposure opportunity for at least some types of military units and their personnel.
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The committee also notes that while such adjustments offer the potential to provide more local detail to measures of exposure opportunity, the geographic size of the grid cells underlying the GIS presents a practical limit to the ultimate spatial resolution of any such adjustments. TCDD Exposure In its current form, the Stellman team's model does not offer means of generating exposure scores linked specifically to TCDD.
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To investigate the range of potential exposures among Vietnam veterans, the Stellman team identified 1,957 "stable" Army units in Vietnam in June 1969 and the 2,095 cells in the GIS grid that these units occupied during that month (Stellman et al., 2003b)
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Both hits and the EOI have merits, but both are likely to cause some level of exposure misclassification. Sensitivity Analyses Needed Given the nature of the proximity-based exposure opportunity measures and the associated assumptions regarding decreases in herbicide concentration with distance and time, the committee stresses the importance of quantitative assessments of the sensitivity of these measures to the model assumptions.
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From page 53... ...
is used instead, it is then desirable to compare the surrogate with other exposure indicators. In the case of the Stellman team's exposure opportunity model, correlation of the EOI with variations in TCDD levels in independent environmental data or in tissue samples could theoretically increase confidence in the model's measures of exposure opportunity for the herbicides that contained the TCDD contaminant.
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. Therefore sediment analysis could only be used to validate the estimates of exposure opportunity generated by the Stellman team's model on very broad geographic and temporal scales.
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measured TCDD levels in serum samples from 646 enlisted Vietnam veterans with a pay grade between E1 and E5 and only one tour of duty in Vietnam (average 320 days) who served in one of five combat battalions in III Corps during 1967–1968.
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From page 56... ...
The analysis used this exposure measure as well as two others, all of which were based on proximity in space and time to Agent Orange spraying, as the Stellman team's measures are. Although a range of TCDD levels was found in this study, the serum levels of those who served in Vietnam, which ranged from non-detectable levels to 45 ng/g lipid, were not significantly different from the levels among veterans with no Vietnam service (range: non-detectable levels to 25 ng/g lipid)
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TESTING AND REFINING THE STELLMAN TEAM'S MODEL The Stellman team's model for herbicide exposure in Vietnam counts direct exposure events and also produces a quantitative representation of
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. Other factors such as the types of vegetation, characteristics of the initial and remaining canopy, and meteorological parameters that could affect the ground-level deposition, photodegradation rate, and the availability of herbicide in the topsoil could also be incorporated into a more detailed exposure model that might use the Stellman team's EOI or the spraying location data in the GIS as a starting point.
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should be examined in the same way. CONCLUSIONS Based on its review of the Stellman team's herbicide exposure assessment model, the committee reached several conclusions.
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4. Fate and transport processes not incorporated into the current version of the Stellman team's model (e.g., width of the spray swath, concentration of contaminants, primary and secondary drift, soil conditions, initial and remaining canopy, and photodegradation)
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1989. Comparison of serum levels of 2,3,7,8-tetrachlorodibenzo-p-dioxin with indirect estimates of Agent Orange exposure among Vietnam veterans: Final report.
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West Vancouver, British Columbia, Canada. HEA-V (Herbicide Exposure Assessment–Vietnam)
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2004. Exposure opportunity models for Agent Orange, dioxin, and other military herbicides used in Vietnam, 1961–1971.
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1994. Correlation between dioxin levels in adipose tissue and estimated exposure to Agent Orange in South Vietnamese residents.
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