The committee’s charge consisted of two tasks:
- Evaluate the reliability (including representativeness, consistency, methods used) of the available information for establishing exposure.
- 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.
Some of the desired types of information (particularly ample sampling information from the time of the Reservists’ exposure) that any exposure assessor would prefer to have when addressing these tasks simply do not exist. Similarly, much other important information is not available in any definitive form. The committee did its best to integrate the information provided by the military through the US Department of Veterans Affairs (VA), from the retired Air Force (AF) Reservists, and by other interested parties. Relevant publications from the peer-reviewed literature were consulted, but a great many of the critical documents (for example, personal statements, letters, memos, and commissioned reports) fall into the category of documents that have not been peer-reviewed, sometimes referred to as “grey literature,” and so must be regarded as being somewhat less than authoritative.
In numerous instances, information and opinions from various sources (documents and people) differed considerably. Controversy about many issues persists with no definitive way to establish what the facts are. The committee is not in a position to make final judgments surrounding discrepancies between the recollections of military personnel and those of the AF Reservists. In some instances,
the resolution of these very heated debates would make very little difference in the execution of the committee’s task. For instance, this is the case for the dispute about what efforts were made to decontaminate the Operation Ranch Hand (ORH) C-123s before their use by the Reservists or even what decontamination measures were taken by the Reservists themselves. No matter what methods may have been used, TCDD and phenoxy herbicide residues were still detected 30 years later in several of the C-123 aircraft at levels in excess of international guidelines. The committee anchored its deliberations upon the facts available for incorporation in the scientific endeavor of exposure estimation and did not give particular credence to any party’s recollection of the events.
The committee notes that the two bulleted tasks in its charge are followed by
The possible health effects would be assumed to be those characterized in prior Veterans and Agent Orange reports, and would not be re-assessed for this report. VAO activities to date have found the information concerning the exposure of Vietnam Veterans inadequate to establish dose-response relationships for individual health outcomes or to quantify the risk of a particular Veteran experiencing any adverse effect.
This committee does concur with earlier IOM committees responsible for the biennial updates in the Veterans and Agent Orange (VAO) series that there are inadequate data to establish dose–response relationships for particular health outcomes or to quantify the increased risk of any individual veteran to experience any adverse effect. The committee’s task has been to assess whether the AF Reservists using the C-123s that had sprayed herbicides in Vietnam had experienced exposures that might increase their risk of any adverse health outcome. The estimates of toxic potency underlying the guidelines referred to by the committee have been derived from controlled animal studies, rather than the epidemiologic results that underlie conclusions concerning association in the VAO series.
FINDINGS IN RESPONSE TO THE FIRST TASK OF THE COMMITTEE’S CHARGE
Only very limited sampling data were collected from the C-123s, and all but the 1979 herbicide samples from “Patches” were gathered decades after the AF Reservists’ exposures on the aircraft had occurred. The data include a series of spot samples in a subset of the planes used by the Reservists with only some of the sampled aircraft having been used in ORH. The limitations of the data available on the levels of TCDD contamination in the interior of the C-123 planes used between 1972 and 1982 by the AF Reservists include the following:
- The sampling efforts were not designed to quantitatively assess the potential exposure to the AF Reservists.
- TCDD sampling was not contemporaneous with the exposure period of concern, and there is great uncertainty about what changes in TCDD levels had occurred inside the planes, during active use prior to 1982 when depletion due to high air turnover would have been maximal and then during their long storage in the desert.
- 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.
- The limitations of the available information make them inadequate for deriving definitive quantitative estimates of exposure, but they are sufficient for a screening level of analysis. Despite these limitations, it is significant that the interiors of the C-123s that had sprayed herbicides in Vietnam and were later used by the AF Reservists had Agent Orange (AO) and TCDD contamination at levels in excess of international guidelines long after their use by the AF Reserve personnel.
Understanding of the physical and chemical characteristics of semi-volatile organic compounds (SVOCs) like TCDD establishes that they would not be immobilized on surfaces, so residues were available for transfer by dermal contact, inhalation, and ingestion. 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. The committee notes that the sampling results appear to be consistent with redistribution of dioxin in accordance with established thermodynamic principles which predict more uniform readings as contamination in hot spots vaporize and resettle averaging out the concentration over the entire interior surface with the passage of time. The exposure potential of individuals working in that environment was likely highly variable. Even if the sampling results were considered an adequate basis for estimating exposure, the information necessary for derivation of estimates on an individual basis is not available.
Air TCDD levels would be expected to be higher on the ground than while flying, due to lower air circulation. Maintenance workers would, therefore, be expected to have higher inhalation exposures per time spent in the planes than crew
members. Work practices of maintenance workers may also have involved more contact with contaminated surfaces than those of crew members. Consequently, depending on the amount of time maintenance workers spent in planes, their TCDD exposures could have been higher than those of C-123 crew members.
FINDINGS IN RESPONSE TO THE SECOND TASK OF THE COMMITTEE’S CHARGE
- Of the various interpretations of the available data available for review, the committee finds the Agency for Toxic Substances and Disease Registry’s (ATSDR’s) qualitative assessment (2012, 2013a,b, 2014) to be the most reasonable and well-supported.
- How representative the very limited number of TCDD samples gathered from the ORH C-123s are of the TCDD distribution throughout their interior 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 or depletion 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 taking 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 the AF Reservists worked in the planes.
- Because of problems related to factoring dermal absorption into the some guidelines, the committee recognized that several of those referred to during its evaluation might not be as protective as supposed. Several guidelines underestimated the extent to which dermal absorption contributes to total exposure in the workplace. In another instance, a guideline for surface loading purported to be protective for the combination of dermal and oral exposure was calculated erroneously so that the more sensitive dermal pathway effectively had not been factored in.
- The committee did not find any of the existing contamination guidelines for TCDD it reviewed or the three models as presented and parameterized in Lurker et al. (2014) a perfect match for the circumstances being evaluated, but it did decide that the surface Toxicity Equivalency Quotient (TEQ) loading guidelines were most applicable to the AF Reservist’s occupational situation.
- The committee switched its attentions to the guidelines themselves after determining that efforts to adjust the poorly documented work profiles of
the Reservists to correspond with those hypothesized for office workers in deriving the guideline had an indeterminate effect. 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 accord with international guidelines.
- The existing guidelines for TEQs on interior surfaces ranged from 1 to 25 ng/m2, demarcating a zone in which sampling measurements reach a level where action should be taken.
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.
The available sampling data are sufficient to demonstrate long-term Agent Orange and TCDD contamination of the C-123s. Understanding of the physical and chemical properties of TCDD establishes that residues measured on the inorganic surfaces within the C-123s would not have been immobile and that contact with the exterior of the AF Reservists’ bodies would have occurred to some extent. Retrospective estimation of concentrations present at the time of the AF Reservists’ service based on these limited measurements would inevitably be subject to substantial uncertainty. At the time when the AF Reservists were working on these C-123s, the levels of TCDD would have been at least as high as those measured at later times. Hence, exposure estimates based on the collected samples without adjustment for any depletion of the TCDD loading on interior surfaces would likely underestimate the actual exposure experienced many years earlier. Direct comparison of those surface loading measurements with existing TCDD guidelines without additional adjustments to incorporate assumptions about work practices during stateside use of the ORH C-123s would not be expected to systematically overestimate their exposures and associated risks.
Bearing in mind all of the factors discussed above, the committee reached consensus that it is probable that the TCDD exposures of at least some AF Reservists exceeded levels equivalent to some guidelines established for office workers in enclosed settings. The committee’s interpretation of the available data is that, although they do not permit definitive quantitative estimation of exposure due to a multitude of uncertainties, they do indicate that it is plausible that the C-123s did contribute to some adverse health consequences among the AF Reservists who worked in ORH C-123s after the planes returned from Vietnam. The committee is firm in its conviction that the AF Reservists working in ORH C-123s were exposed (in the technical sense of the word of having bodily contact with the chemicals) to the components of AO to some extent. The committee members could not stand behind any particular exposure estimates produced by manipulating the existing data, but they are clear in their finding that the surface-
wipe sampling measurements of dioxin gathered in 1994, 1995, and 2009 are fully consistent with exposures to the AF Reservists while working in ORH C-123 planes that exceeded international exposure guidelines.
Of the ORH C-123s returned to the United States from Vietnam, most have been destroyed. The exceptions are the now decontaminated C-123 at Wright-Patterson Air Force Base and two other C-123s that did go to Vietnam and most probably sprayed herbicides (although documentation remains lacking) are on display at military aviation museums in Delaware and Georgia (the public is not permitted access to their interiors). Efforts to recover the work records of the AF Reservists have been unsuccessful, so it is highly unlikely that any additional information will become available to establish more definitively the magnitude of exposures experienced by the AF Reservists.