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Characterizing Exposure of Veterans to Agent Orange and Other Herbicides Used in Vietnam: Scientific Considerations Regarding a Request for Proposals for Research synthesis of a variety of information sources, because information on locations, operations, and activities may be combined to define relative differences in exposure. AVAILABLE DATA There are sources of publicly available data on the use of herbicides in Vietnam, veterans' service in the Vietnam theater, and characteristics of other populations exposed to dioxin that may be of use to researchers. Some of these sources are mentioned below or are cited in the Appendix. However, their mention here is for information only and is not intended as a recommendation for their use. Neither should this listing be considered comprehensive. Data regarding individuals may be subject to privacy restrictions. The HERBS and Service HERBS computer tapes contain information regarding aerial and some ground-spraying activities, including date, type of herbicide, number of gallons sprayed, and location of spraying. These tapes and their limitations are discussed in greater detail in the Appendix and in Veterans and Agent Orange (IOM, 1994:96–106). Data from the 1987 Air Force Health Study physical examination of Ranch Hands veterans and a comparison group is available through the National Technical Information Service (NTIS Accession Number A-283-319). These data include date of birth, exposure group, and dioxin levels for approximately 2,500 subjects. The Minnesota Department of Veterans Affairs has compiled service information on approximately 156,000 Vietnam-era veterans—including approximately 68,000 who served in Vietnam—in support of that state's bonus program. The DVA is developing a comprehensive Vietnam veterans roster that will include service branch, rank, occupational codes, and the dates of Vietnam service. This roster is expected to be completed during 1997. The National Institute of Occupational Safety and Health has collected information on 5,172 male workers at 12 plants that produced TCDD-contaminated herbicides. Among these data are vital statistics and duration of exposure; lipid-adjusted, serum TCDD levels are also available for a subset of 253 workers. POTENTIAL APPROACHES Exposure reconstruction should consider methods that will identify homogeneously exposed groups, rank-order cohorts by exposure levels, or use some combination of approaches to generate information for epidemiologic studies. This could include, but is not limited to, using biomarkers of exposure; identifying exposure patterns (e.g., from air vs. ground spraying), locations, concentrations, and frequencies; considering types of foliage, weather conditions, troop location and movement in time; date(s) of field service for
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Characterizing Exposure of Veterans to Agent Orange and Other Herbicides Used in Vietnam: Scientific Considerations Regarding a Request for Proposals for Research individual soldiers; and distinguishing the types of herbicides used. The section entitled ''Development of the Exposure Reconstruction Model'' in the Appendix (p. 46) discusses this in greater detail. Biomarkers of exposure require the ability to measure a chemical in a biologic specimen—that is, urine, blood, tissue, organs, hair, and the like. If biomarkers of exposure are chosen as a dose index, such as serum TCDD, then the quantitative relations between the marker and herbicide exposure and its limitations must be discussed. For example, individual clearance rates of TCDD are variable, so low serum TCDD may not equate to low past exposure. There are also other sources of TCDD intake, such as diet, so current high TCDD levels may not necessarily indicate high past exposure. Finally, TCDD was a highly variable constituent of the herbicides used in Vietnam, and some herbicides did not contain TCDD; therefore, TCDD may not be a good marker for all types of herbicide exposure. Additional information on biomarkers is provided in the Appendix. VALIDATION OF EXPOSURE ASSESSMENT APPROACHES Validation of the exposure assessment approach developed will be a critical part of the proposal. The committee recognizes that retrospective exposure assessment of herbicide exposures among Vietnam War veterans may, at best, be semiquantitative or rank ordered. Because we have a generally poor understanding of the sources and magnitudes of exposures to herbicides that occurred in Vietnam more than 25 years ago, the predicted levels of exposure to individual veterans are likely to be imprecise. Thus, applicants should discuss the anticipated sources and magnitudes of uncertainty in their model predictions. Attention should also be paid to the role that quantitative or semiquantitative data might play in the validation of exposure predictions in a pilot study. The best method for validation is not obvious and is left to the researchers to propose, critique, and defend. The limitations and uncertainties associated with the validation method—such as those discussed previously for serum TCDD levels—should be addressed. Respondents are encouraged to consider other alternative approaches to the validation process. EVALUATION CRITERIA The purpose of the research to be sponsored under the RFP is to develop exposure assessment models for use in epidemiologic studies of Vietnam veterans. Proposals of models for which the relevance to Vietnam veterans is unclear will not be further evaluated. The proposals will be assessed based on
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