7SUMMARY AND CONCLUSIONS

PRIOR EXPOSURE ASSESSMENTS

Since the 1970s, IOM committees and other groups have attempted to determine or reconstruct Vietnam veterans’ potential exposure to Agent Orange and TCDD. In particular, two IOM committees assessed an exposure-opportunity model developed by Columbia University principal investigator Jeanne Stellman (IOM, 2003, 2008). The Stellman model is based on the Department of Defense Herbicide Reporting System files on Operation Ranch Hand spraying missions and military personnel records of veterans who served on the ground in Vietnam, including Army, Marine, Navy, and Air Force personnel. The model assigned an exposure-opportunity index value to individuals or groups that had homogeneous exposure characteristics (location and time) that were based on proximity to flight paths of aerial spraying missions and time concordance between spraying and presence in the affected areas. Full application of the exposure-opportunity model to ground troops in Vietnam was never accomplished and has never been proposed for Brown Water Navy or Blue Water Navy personnel.

The suitability of the model for classifying ground troops by degree of probable exposure for the purposes of epidemiologic study has been the subject of some discussion and analysis. The IOM report The Utility of Proximity-Based Herbicide Exposure Assessment in Epidemiologic Studies of Vietnam Veterans (IOM, 2008) concluded that the Stellman approach was reasonable, that the model for all its approximations held promise for developing exposure information suitable for epidemiologic studies of Agent Orange health effects, and that the model should be implemented by the Department of Veterans Affairs (VA). The application of the model for units or groups, even with very large-scale distinctions in time or location, has not been attempted. Other investigators have identified diferences in predicted ground-level spray deposition in the Stellman model compared with established pesticide-spray drift models, such as AgDRIFT. The lack of accuracy, precision, and completeness in troop-location data has also been identified as a limitation that would prevent the Stellman approach from being applied reliably to epidemiologic studies (Ginevan et al., 2009a,b). However, alternatives to the Stellman exposure-opportunity model have not been proposed. Concentrations of Agent Orange–associated TCDD in the Vietnamese environment during or shortly after the war or the number of military troops that might have been exposed to Agent Orange–associated TCDD have not been determined.

LIMITATIONS OF AND UNCERTAINTIES IN THE AVAILABLE INFORMATION

Numerous limitations and uncertainties are associated with the information that the committee used to come to its conclusions. Chief among them is the lack of environmental monitoring data on TCDD in Vietnam during the war. The committee relied on previous IOM reports (IOM, 1994, 2003, 2008, 2009) for an assessment of potential exposure of ground troops to TCDD, but those reports did not include exposure assessments for Brown Water Navy or Blue Water Navy personnel. Lack of information on the Brown Water Navy or Blue Water Navy activities, on the use of Agent Orange, and on environmental monitoring contributed to the uncertainties surrounding their potential exposure to TCDD. The committee accepted the conclusions of previous IOM committees that evaluated the utility of the Stellman exposure-opportunity model; however, the value of that model to the present committee’s charge was minimal in that no exposure values had been generated.

The committee did not believe that its charge included a review of the VA’s current approach to determining whether a Blue Water Navy veteran had been exposed to Agent Orange as that is a policy decision by the VA. The committee notes, however, that for a Blue Water Navy sailor to be eligible for compensation, the sailor or the VA must show that the sailor’s ship meets the criteria for classification as a Brown Water Navy vessel during a specified period and that the sailor was aboard the ship at this time. That is a time-consuming task for both the Navy veteran and the VA in that it requires a review of each ship’s deck logs.

Finally, the committee spoke with and received information from numerous Vietnam veterans about their war experiences. That information was helpful in providing the committee with an appreciation of the activities of Blue Water Navy personnel (and other veterans) during the war, but some of the information was contradictory, and confirmation of specific events was not possible or was unavailable to the committee. Although records of such events as spray missions were kept, the committee recognizes that events could occur during missions that would alter the flight path of an approved mission (for example, there may have been enemy fire, mechanical problems may have resulted in the spraying of more or less herbicide, and the mission may have been aborted or the herbicide dumped); some such alterations may be recorded, but it is possible that the records are not entirely accurate. The committee greatly appreciated each veteran’s input but notes the difficulty in interpreting and using this type of information in assessing population exposure.

The committee notes that Blue Water Navy personnel were also exposed to numerous chemicals during active duty that are known to exert chronic adverse health effects. However, as with TCDD exposure, there were no quantitative measurements of the environmental concentrations of those chemicals aboard US Navy ships during the war. During its tour of the USS Midway, the committee noted that Blue Water Navy personnel could have been exposed to a mixture of chemicals on the ships including polychlorinated biphenyls and asbestos, among other hazardous chemicals. Given that a tour of duty was typically 1 year, it is possible that the Navy personnel were exposed to a variety of hazardous chemicals, including chemicals used for occupational purposes, such as solvents and fuels, and those from the ubiquitous cigarette smoke, during the entire period.

Another approach to determining whether Blue Water Navy personnel may have been exposed to TCDD would be to assess the incidence of TCDD-associated adverse health effects in this group of veterans and compare it with the incidence of the same adverse health effects in ground troops and Brown Water Navy personnel. A similar pattern of incidence might be evidence of comparable exposure, and a higher incidence might indicate greater exposure. However, such incidence data are not available on Blue Water Navy personnel. With few exceptions, Blue Water Navy personnel have not been distinguished as a separate cohort in epidemiologic studies of Vietnam veterans. Therefore, the committee was unable to determine, using this approach, whether Blue Water Navy personnel had a greater or smaller potential for exposure to TCDD and for developing adverse health effects. Studies of Royal Australian Navy Vietnam veterans were inconsistent with US studies that associated TCDD exposure with non-Hodgkin’s lymphoma. Many of the studies do not account for the high prevalence of cigarette smoking or possible exposure to secondhand smoke aboard naval vessels during the Vietnam War. The conflicting information on the prevalence of non-Hodgkin’s lymphoma in Australian versus American Vietnam veterans suggests that further follow-up on the health of the latter population, particularly for Blue Water Navy veterans who had considerable opportunity for exposure to numerous toxic chemicals during their tours of duty, is warranted.

CONCLUSIONS

The committee first approached its task by attempting to estimate the concentrations of TCDD that might have been in the coastal waters of Vietnam as a result of the spraying of Agent Orange during Operation Ranch Hand. This information could then be used to determine possible exposure mechanisms and exposure opportunities of Blue Water Navy personnel. A search of the literature indicated that no measurements of TCDD in any environmental medium in Vietnam had been made during the war or shortly enough after the war to provide reasonable estimates of environmental concentrations. Therefore, the committee explored a modeling approach that used fate and transport considerations in conjunction with multimedia dioxin models developed for other purposes. However, those models are complex and require extensive input data on the Vietnamese environment pertinent to the time of Agent Orange release as well as data on the characteristic behavior of TCDD in that environment. Input parameters are subject to extensive uncertainty, and model results cannot be evaluated for their veracity. Therefore, the committee was not able to estimate even a range of environmental concentrations of TCDD that might be present in Vietnamese coastal waters or the amount of TCDD that might reach Blue Water Navy ships as a result of spray drift from Operation Ranch Hand missions flown near the coast.

With regard to exposure modeling, the committee recognized that despite a substantial investment of time and resources for determining the potential exposure of Vietnam veterans who had served on the ground to Agent Orange–associated TCDD, no actual exposure values have been determined. Nevertheless, the committee did consider whether worst-case scenarios might be helpful in estimating potential exposures for Blue Water Navy veterans. The committee did attempt to do some “back of the envelope” calculations but determined that the uncertainties in the input parameters such as concentrations of TCCD in marine water, ship locations, and dates and locations of spraying missions near the coast, invalidated any exposure estimates. Furthermore, the committee believed that any worst-case estimates might be taken out of context and might be misconstrued as actual exposure estimates. Given the lack of exposure data on ground troops and the knowledge that Brown Water Navy and Blue Water Navy veterans had not even been considered in such exposure-opportunity modeling, the committee concluded that it was not possible to make quantitative exposure comparisons among the three military populations of interest to the VA nor was it possible to do any worst-case bounding of exposure estimates. Therefore, the committee approached the task by evaluating the plausibility of exposure of the three populations to Agent Orange and TCDD via various mechanisms and routes.

Even with the lack of quantitative exposure information, the committee recognized that considerable variability exists in the potential for TCDD exposure of Blue Water Navy personnel and ground troops and Brown Water Navy personnel. It is possible that some fraction of Blue Water Navy personnel were not exposed to Agent Orange–associated TCDD, either directly or indirectly, but the proportion of personnel for whom that would have been the case is not known nor is it estimable with available information; there are similar uncertainties in estimating exposure of ground troops and Brown Water Navy sailors and the proportion of those personnel who might have been exposed. Some Blue Water Navy personnel may have spent their entire tour of duty on aircraft carriers that never came close to the Vietnamese coast, and others served on ships (for example, destroyers) that may have spent many days as close as a mile offshore. In addition to the variability in the location of Blue Water Navy personnel, there is extensive uncertainty regarding the experiences of individual sailors on those ships (for example, whether they were on a ship when it was near the Vietnamese coast, whether they went swimming, and whether they ate local food from Vietnam or Vietnamese waters).

The committee concludes that, qualitatively, ground troops and Brown Water Navy personnel had more pathways of exposure to Agent Orange–associated TCDD than did Blue Water Navy personnel (see Figure 5-1). One exposure mechanism is specific to Blue Water Navy ships: possible TCDD contamination of potable water from shipboard distillation plants. The committee’s assessment corroborates the Australian finding that in experiments simulating the water-distillation system used on Navy ships the system had the potential to enrich TCDD concentrations from the feed water to the distilled potable water. However, without information on the TCDD concentrations in the marine feed water, it is impossible to determine whether Blue Water Navy personnel were exposed to TCDD via ingestion, dermal contact, or inhalation of potable water.

After examining a wealth of information on possible routes of exposure, the committee concluded that it would not be possible to determine Agent Orange–associated TCDD concentrations in the Vietnamese environment. This lack of information makes it impossible to quantify exposures for Blue Water and Brown Water Navy sailors and, so far, for ground troops as well. Thus, the committee was unable to state with certainty whether Blue Water Navy personnel were or were not exposed to Agent Orange and its associated TCDD. Moreover, the committee concluded that it could not state with certainty that exposures to Blue Water Navy personnel, taken as a group, were qualitatively different from their Brown Water Navy and ground troop counterparts. Indeed, the committee felt that the paucity of scientific data makes it impossible to determine whether or not Blue Water Navy veterans were exposed to Agent Orange–associated TCDD during the Vietnam War.

REFERENCES

• Ginevan, M. E., J. H. Ross, and D. K. Watkins. 2009. a. Assessing exposure to allied ground troops in the Vietnam War: A comparison of AgDRIFT and Exposure Opportunity Index models. Journal of Exposure Science and Environmental Epidemiology 19(2):187-200. [PubMed: 18335003]

• Ginevan, M. E., D. K. Watkins, J. H. Ross, and R. A. O’Boyle. 2009. b. Assessing exposure to allied ground troops in the Vietnam war: A quantitative evaluation of the Stellman Exposure Opportunity Index model. Chemosphere 75(11):1512-1518. [PubMed: 19278712]

• IOM (Institute of Medicine). 1994. Veterans and Agent Orange: Health effects of herbicides used in Vietnam. Washington, DC: National Academy Press. [PubMed: 25144022]

• IOM. 2003. Characterizing exposure of veterans to Agent Orange and other herbicides used in Vietnam. Washington, DC: The National Academies Press.

• IOM. 2008. The utility of proximity-based herbicide exposure assessment in epidemiologic studies of Vietnam veterans . Washington, DC: The National Academies Press.

• IOM. 2009. Veterans and Agent Orange: Update 2008. Washington, DC: The National Academies Press. [PubMed: 25032343]