From 1972 to 1982, approximately 1,500–2,100 US Air Force (AF) Reserve personnel trained and worked on C-123 aircraft that had formerly been used to spray herbicides in Vietnam as part of Operation Ranch Hand (ORH). After becoming aware that some of the aircraft on which they had worked had previously served this purpose, some of these AF Reservists applied to the US Department of Veterans Affairs (VA) for compensatory coverage under the Agent Orange (AO) Act of 1991. The AO Act provides health care and disability coverage for health conditions that have been deemed presumptively service-related for herbicide exposure during the Vietnam War. The VA denied the applications on the basis that these veterans were ineligible because as non-Vietnam-era veterans or as Vietnam-era veterans without “boots on the ground” service in Vietnam, they were not covered by the AO Act. However, with the knowledge that some air and wipe samples taken between 1979 and 2009 from some of the C-123s used in ORH showed the presence of AO residues, representatives of the C-123 Veterans Association began a concerted effort to reverse VA’s position and obtain coverage.
In early 2014, the VA contracted with the Institute of Medicine (IOM) to evaluate whether or not service in these ORH C-123s could have plausibly resulted in exposures detrimental to the health of these AF Reservists. The IOM was asked to assemble an expert committee to address this question qualitatively, but in a scientific and evidence-based fashion. The resulting committee was explicitly directed that its role was not to make any policy determinations concerning
the applicability of the AO Act to this group. Specifically, the committee was charged to
- evaluate the reliability (including representativeness, consistency, methods used) of the available information for establishing exposure; and
- address and place in context (qualitatively by comparison to established exposure guidelines) whether any documented residues represent potentially harmful exposure by characterizing the amounts available and the degree to which absorption might be expected.
The possible health effects associated with these exposures would be assumed to be those characterized in prior IOM Veterans and Agent Orange (VAO) reports and were not to be reassessed for this report.
THE COMMITTEE’S APPROACH
At its initial meeting, the committee decided to undertake several activities in three general areas in order to complete its tasks.
- Organize the available information, identify gaps and inconsistencies, and endeavor to resolve such issues.
- Screen for relevance and categorize the documents provided.
- Gather information on existing health guidelines and their derivations.
- Evaluate the sampling procedures and results.
- Conduct a workshop to clarify issues and to obtain missing information.
- Evaluate existing information and determine what interpretations regarding exposure were supported.
- Consider various interpretations of the available data.
- Review existing exposure estimation models.
- Evaluate whether the detected AO residues could have reached the AF Reservists’ bodies and then been absorbed.
- Establish plausible magnitude of the AF Reservists’ exposure.
- In the context of existing guidelines for 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD or dioxin) and dioxin-like compounds (the toxic contaminants of some of the herbicides sprayed in Vietnam), assess whether the magnitude of exposure supported by the existing information could plausibly be associated with adverse health effects during the time period when the AF Reservists were potentially exposed.
How the committee went about performing these activities is elaborated in Chapter 1, while the results of those efforts are summarized below.
THE AVAILABLE INFORMATION
The committee evaluated an assortment of documents submitted by the VA, veterans, and other interested parties, and collected relevant data from published journal articles and technical guidelines developed by authoritative organizations. The VA provided the committee with the sparse sampling data available for the C-123 aircraft and all relevant associated background information on the history of the planes’ use and sampling, plus several military interpretations of these data and an extensive set of possibly pertinent documents. The C-123 Veterans Association and other interested parties provided historical records, personal accounts of service aboard C-123s, procedure manuals, aircraft logs, and some flight information from Air Squadrons and AF Reserve bases. The committee obtained a considerable amount of additional information, particularly on technical matters and guidelines from authoritative bodies, from the peer-reviewed literature. It also scanned a considerable amount of non-peer-reviewed material available on the Internet for clues that might lead to more definitive sources. The committee also held a public workshop to gather additional information and to hear from veterans and veterans’ service organizations, representatives from the VA, researchers, and other interested parties.
Some fundamental information about the exposure of the AF Reservists was either not recoverable at all or the content provided by various sources could not be reconciled. For instance, considerable effort has failed to establish exactly how many C-123s the military had in Vietnam; how many of them for spraying insecticides; how many were used for spraying herbicides (the ones referred to in this report as ORH C-123s), how many were returned to the United States, how many ORH C-123s and how many C-123s that had not been in Vietnam were allocated to the various reserve units; and how many AF Reservists possibly worked in ORH C-123s.
As would be expected as a consequence of photolysis with exposure to sunlight and non-containment of volatilized chemicals, when several C-123s were finally sampled, their exteriors were not demonstrably contaminated with residues of TCDD or the herbicides sprayed in Vietnam. Only a very small number of samples for measurement of TCDD and the herbicides in AO were taken from the interior surfaces and from the air in a small number of the ORH C-123s flown by the AF Reservists (see Table S-1 for an inventory).
At the time of the 1979 sampling, methods of TCDD analysis were not readily available and had limited sensitivity, but TCDD is generally recognized as being more toxic than the herbicides, which are also less persistent than this contaminant. The committee, therefore, considered TCDD to be the component of the ORH-derived chemicals most relevant for assessing health risk. The short
TABLE S-1 Summary of Interior Sampling Efforts Conducted on C-123 Aircraft Previously Used to Spray Herbicides in Operation Ranch Hand
|Year||TCDD||2,4-D 2,4,5-T||TCDD||2,4-D 2,4,5-T|
|1979||2 from A||3 from A|
|1994||3 from A|
|1995||5 from A|
|1996||2 from unknown (not usable)||2 each from B, C + 10 more|
|2009||7 from B, 9 from C||7 from B, 9 from C||1 each from B + C||1 each from B + C|
NOTES: Number of interior samples taken from specific C-123s with documented use in Operation Ranch Hand.
C-123 tail numbers: A = #56-4362 (i.e., “Patches”); B = #55-4532; C = #55-4571.
periods over which the two TCDD air samples were gathered make them of questionable reliability, so the committee focused its qualitative assessment on the interior TCDD wipe samples collected from a total of three ORH C-123s. The two interior surface samples without information on area sampled could not be converted into usable results. The 24 usable measurements are graphed in Figure S-1.
EVALUATION AND INTERPRETATION
The long delay between when the exposures occurred (1972–1982) and when the measurements were made (1979 for herbicides only and not until the mid-1990s and 2009 for TCDD) is a serious limitation of the sampling data. Over this long period, degradation and loss processes were occurring to an unknown extent, so the levels when the Reservists were exposed would have been at least as high as the measurements made later. The lack of knowledge about decreases in TCDD and herbicide concentrations between the AF Reservists’ time in the C-123s and later sampling precludes any attempts to make adjustments for a predicted decay in concentration. The delay before sampling contributes additional uncertainty to any quantitative estimates made of exposure, while very likely biasing the results toward underestimates. There also is uncertainty about the fraction of the C-123s worked in by the AF Reservists that had actually been used to spray herbicides in ORH, in addition to very incomplete knowledge of the AF Reservists’ profiles of work that would be needed to estimate the contact rate, frequency, and time in quantitative modeling.
FIGURE S-1 Existing TCDD surface concentrations in relation to interior wipe samples.
NOTES: TCDD surface concentrations obtained from the total of 24 wipe samples from the interiors of ORH C-123 aircraft by year. The horizontal band represents the 1–25 ng/m2 range of existing surface guidelines for TCDD.
Clear points represent non-detect samples plotted at their detection limit. The concentration of TCDD in four samples gathered from “Patches” in 1994 were below their limits of detection, which in turn were not sensitive enough to evaluate whether or not the samples’ concentrations were less than 1 ng/m2 (below the range of concern).
The committee assessed arguments presented in previous interpretations of the available information on AO contamination of the C-123s used by the AF Reservists from 1972–1982. It noted that those from in the military or associated with the VA tended to minimize the possibility of an increased risk of exposure and adverse health outcomes among the AF Reservists. A recurrent theme in these assertions was that dry AO residues containing TCDD detected on interior surfaces of ORH C-123s long after the planes returned from Vietnam could not have moved from the surfaces and thus were unable to complete transfer to the “outer boundary of a human” to constitute exposure as defined by the US Army’s Center for Health Promotion and Preventive Medicine. The committee notes emphatically: it is now accepted in the field of exposure science that the physicochemical properties of semi-volatile organic compounds (SVOCs) like TCDD keep them in dynamic flux toward equilibrium in enclosed spaces among the various phases present (liquid, gas in air, airborne particles, dust on surfaces, residues or films on surfaces).
The committee explored the range of exposure estimates generated when plausible scenarios were used as input for the various exposure models proposed in the reviewed literature for estimating potential exposure from the available data. To put such findings in context, they were compared to existing guidelines for TCDD exposure within enclosed settings. When making such comparisons, the committee was contrasting the range of values it had adopted as being realistic for the AF Reservists for the variables in the models (such as frequency and duration of exposure in the contaminated environment, contact with surfaces, and breathing rates) to the extreme values assumed for the office worker population for which the guidelines had been developed (for example, 30 or 40 years working in a contaminated office). Compared to office workers, the AF Reservists’ engaged in more vigorous movements and activities with greater exposure potential (such as having more contact with additional surfaces, sitting on the floor of the aircraft, having limited or no access to lavatory facilities, eating in the contaminated area), which suggests they would have experienced higher exposure from inadvertent ingestion and dermal pathways than office workers. On the other hand, the number of hours and years that the AF Reservists worked in the aircraft were less than what was assumed in establishing guidelines for the office workers. Setting aside efforts to equate the quite uncertain work profiles of the AF Reservists to the extreme work patterns assumed when deriving guidelines intended to apply to all office workers, the committee regarded the assembled existing guidelines for surface loading, which ranged from 1 to 25 ng/m2, as a rough basis for assessing the set of sampling measurements presented in Figure S-1.
The committee found that none of the exposure models considered incorporated the full spectrum of plausible exposure routes. As a result, these exposure models would generate underestimates of actual exposure when “best” or plausible exposure estimates were sought. Similarly, by overlooking plausible routes of exposure, existing guidelines may overstate the level of protection their values actually provide. Another variant of this problem arose in a 2009 report prepared for Hill Air Force Base that derived screening guidelines for TCDD and the phenoxy herbicides that were intended to take into account both oral and dermal routes of exposure; the committee found that use of an erroneous formula had resulted in screening standards purportedly for oral and dermal exposure that effectively discounted the more sensitive dermal route. This standard was used in several subsequent interpretations of the C-123 TCDD sampling data under the assumption that it was more protective or stringent than was the case, thereby understating associated health risks.
In exploring exposure estimates consistent with the observed sampling results, the committee considered plausible, rather than “worst-case,” scenarios. Having considered a variety of approaches for interpreting the available data and having established the intrinsic weaknesses of those data for quantitative exposure estimation, the committee was unable to determine which, if any, of the various models and exposure scenarios it investigated were most representa-
tive of the experiences of the AF Reservists. The committee observed, however, that, under at least some of the scenarios, all the quantitative models generated exposure estimates for the C-123 personnel that were larger than what screening guidelines deemed to be “acceptably” safe.
When putting its perceptions of the available surface sampling measurements in context by comparison to existing protective guidelines, however, the committee did proceed in accord with the public health practice, often referred to as the “precautionary principle,” that seeks to identify possibly dangerous situations and to provide warning about health before, or at levels below where a problem is evident. Factors contributing to uncertainty discussed in this report (perhaps most importantly the long delay between when the activities leading to possible exposure occurred and sampling, and the guidelines’ failure to account for the extent of dermal absorption) would mean that the measured TCDD surface levels would in all likelihood understate the risk of adverse health effects to which the AF Reservists actually had been exposed.
Because these individually uncertain perspectives on the situation consistently supported the possibility of health risks, the committee concluded that the available information supports the expectation that the health of some of the personnel was adversely affected by their service in the C-123s that had earlier been used to spray herbicides in Vietnam.
THE COMMITTEE’S FINDINGS
Assessment of Available Information
- The sampling efforts were not designed to quantitatively assess the potential exposure to the AF Reservists.
- The fact that TCDD sampling was not contemporaneous with the period when the AF Reservists experienced exposure complicates efforts to use the results to assess the possibility that harm to their health may have occurred.
- Sampling and analysis methods used over the various sampling periods apparently were not uniform, but the methodologies were not fully described. Aside from the air samples which were collected using inappropriate methods, however, the committee did not find information to invalidate any of the reported measurement data.
- Considerable nonuniformity in the distribution of contamination throughout the interior of “Patches” and differences in sampling procedures may have contributed to the inconsistencies noted between the sampling results in 1994 and 1995.
- Detailed, reliable information is not available on the activities of aircrews and maintenance personnel inside these airplanes (e.g., time spent in
planes, contacts with surfaces, use of protective equipment, etc.) and very little information is available on the use of specific aircraft.
Interpretation of Available Information
- The limitations of the available information make the data inadequate for deriving definitive quantitative estimates of exposure, but they are sufficient for a screening level of analysis.
- The interiors of the C-123s that had sprayed herbicides in Vietnam and were later used by the AF Reservists had AO and TCDD contamination persisting long after their use by the AF Reserve personnel.
- Understanding of the physical and chemical characteristics of SVOCs like TCDD establishes that they would not have been immobilized on surfaces, so residues were available for transfer by dermal contact, inhalation, and ingestion. The AF Reservists serving in the contaminated C-123s, therefore, experienced some degree of exposure to TCDD and herbicides through multiple routes when working in ORH C-123s.
- How representative the very limited number of TCDD samples gathered from the ORH C-123s are of the TCDD distribution throughout their interiors is uncertain, but, in the absence of definitive information to the contrary, the committee assumed that the three ORH C-123s sampled were representative of the entire fleet.
- There is no definitive information on the rate of degradation of TCDD on interior surfaces of the aircraft in the decades after their use in ORH. Without adjustment for reductions in the contamination over time, estimates of TCDD exposures to the AF Reservists based on samples taken from the C-123s in the mid-1990s and in 2009 could therefore underestimate their actual exposures, quite possibly markedly. Therefore, the measurements resulting from interior surface sampling in 1994, 1995, and 2009 probably represent a lower bound on what average surface TCDD contamination might have been when AF Reservists worked in the planes.
- Because of issues concerning inadequate factoring of dermal absorption into the development of guidelines, the committee recognized that several of the guidelines referred to during its evaluation were likely not as protective as might be supposed.
- The committee did not find any of the existing contamination guidelines for TCDD that it reviewed or the three models as presented and parameterized in Lurker et al. (2014) to be a perfect match for the circumstances being evaluated, which (in addition to the committee charge to conduct a qualitative evaluation and the limitations of the available data) represents another impediment to quantitative estimation of the Reservists’ exposures or risks.
- The committee did decide that the surface Toxicity Equivalency Quotient (TEQ) loading guidelines were most applicable to the AF Reservists’ occupational situation. It is the committee’s judgment that comparing the unadjusted surface measurements from the ORH C-123s to the existing guidelines for surface loading provides the most valid qualitative means of evaluating the degree to which these results supported exposures safely less than international regulatory standards.
- The existing guidelines for TEQs on interior surfaces ranged from 1 to 25 ng/m2, a zone in which sampling measurements reach a level where further action would be appropriate.
Although the existing information is inadequate for estimating exposure with any degree of certainty, the committee was able to answer its charge to evaluate the reliability of the data and to qualitatively establish whether the documented residues represent potentially harmful exposures.
Based upon physicochemical principles, the committee rejected the idea that the dioxin residues detected on interior surfaces of the ORH C-123s were immobile and effectively inaccessible to the Reservists as a source of exposure. Accordingly, the committee states with confidence that the AF Reservists were exposed when working in the ORH C-123s and so experienced some increase in their risk of a variety of adverse responses. The committee has shown that results of the most relevant type of sampling (which were collected a very long time after the AF Reservists worked in the ORH C-123s) fall in or above the 1-to-25 ng/m2 range specified as meriting cautionary consideration by international exposure guidelines.
Efforts to recover the work records of the AF Reservists have been unsuccessful. The committee surmises it is unlikely that any additional information will become available to establish more definitively the magnitude of exposures experienced by the AF Reservists.