Post-Vietnam Dioxin Exposure in Agent Orange-Contaminated C-123 Aircraft (2015) / Chapter Skim
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4 Evaluation of Assessments of Possible Exposure of Air Force Reservists from Service in Operation Ranch Hand C-123s
4 Evaluation of Assessments of Possible Exposure of Air Force Reservists from Service in Operation Ranch Hand C-123s
Pages 47-71
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From page 47... ...
. The positions developed were based on qualitative, or at most semi-quantitative, treatment of the sampling data and their sources fall into two general categories: • interpretations by individuals and entities associated with the military (USAF, 1994, 1997b, 2009a,b, 2012a,b; Young and Young, 2012, 2013a)
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From page 48... ...
INTERPRETATIONS OF SAMPLING RESULTS BY ENTITIES ASSOCIATED WITH THE MILITARY Between 1979 and 2009, air and wipe samples were taken from several C-123 aircraft that had formerly been used to spray herbicides in Vietnam and were then used by the AF Reservists in the United States from 1972–1982. These sampling efforts are detailed in Chapter 3 of this report.
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From page 49... ...
The committee has determined that the aggregate screening levels derived from the pathway-specific surface contamination screening levels for the dermal and oral routes shown in Table 4-1 were calculated incorrectly. The USAF report calculated the screening levels by simply adding the individual screening levels for the two routes, rather than by using the inverse-of-the-sum-of-the-inverses formula, which is the US Environmental Protection Agency's (EPA's)
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From page 50... ...
(23 ng/m2) 1 ng/m2 SOURCES: Appendix F: Risk Screening Level Assessment, Table 1 (USAF, 2009b,d)
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From page 51... ...
The USAF review (2012a) also concluded that the air sampling data for the phenoxy herbicides were "within acceptable exposure limits" although no air exposure limits were presented in this document, and it is not clear what exposure limits are being referred to by the authors.
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From page 52... ...
, it is reasonable to conclude that it is not possible to derive quantitative estimates of any increased health risks for those individuals who came into contact with the UC-123 aircraft from 1972 to 1982. The committee finds this paragraph problematic in three respects: • The report's "assessment of risk" could not have been "dependent upon the findings of the National Academy of Sciences' Institute of Medicine RANCH HAND studies" (USAF, 2012a)
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From page 53... ...
In contrast, the issue concerning the AF Reservists and ORH C-123s is whether the risk of adverse health outcomes of any sort was elevated among those with potential exposure. This committee considers the sampling data for the AF Reserve C-123 population, albeit limited, to be an adequate basis for a qualitative exposure assessment and for some judgment about the overall health risk from TCDD exposure that the AF Reservists may have expe rienced, and so finds this justification for not doing so from the authors of the USAF report (2012a)
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From page 54... ...
Kimbrough et al. do not discuss dermal absorption, dismissing it a priori as "a minor contributor to body burdens of the general population." However, dermal exposure can be an important source of exposure in occu ational p settings.
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From page 55... ...
. The initial ATSDR opinion letter acknowledged the limitations of the available data, most notably the questionable representativeness of TCDD surface concentrations from the sampled C-123s to surface levels in the other ORH C-123s and the 20- to 40-year lag time between when the AF Reservists worked on them and collection of the comparison surface wipe samples.
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From page 56... ...
In their June 2014 presentation to the committee, ATSDR additionally pointed out that the newly available sampling data had been collected from wipe samples from different aircraft and sequential sampling of the same surfaces in a given aircraft had not been conducted. Thus, it is not possible to infer degradation rates for TCDD on the sampled surfaces.
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From page 57... ...
The committee calls this the "Dermal-oral Direct Contact Model." Lurker et al.'s (2014) second approach evaluated the potential degree of vapor supersaturation of herbicides in air samples collected in a C-123 in 1979 and then predicted potential inhalation exposures to vapor and particlebound TCDD.
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From page 58... ...
, and to convert units representing surface area (CFa) and expanded the terms for contact rate and for exposure frequency and duration in the original equation as follows: • CR = (SA)
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From page 59... ...
This model is quite representative of commonly used models for exposure estimation, but almost any result could be obtained depending on what values are assumed for the large number of input variables. A broad spectrum of values is usually feasible for any exposure situation under investigation, but the extensive uncertainties about the actual work histories of the AF Reservists in this case make this model even more flexible.
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From page 60... ...
noted that the air concentrations to which the calculated vapor pressures were being compared were not the acid forms of 2,4-D and 2,4,5-T, but rather the esters which have a considerably higher vapor pressure. Based on the premise that the 2,4-D and 2,4,5-T acid vapor concentration calculated was lower than the measured concentration, the measured air concentration would contain both vapor phase and particulate phase of these compounds.
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From page 61... ...
This approach can provide a steady state air concentration in a specified microenvironment with a constant air exchange rate and a source of SVOCs that is not exhausted over the time period of interest. This is an appropriate model to use for TCDD in the post-Vietnam ORH C-123 aircraft if the input parameters can be properly established.
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From page 62... ...
Based on projected on-ground air concentrations, and given that aggregate exposures would be expected to be a multiple of vapor inhalation exposures, average workday exposures to flight crews could be problematic, but maintenance personnel whose on-ground time in the planes exceeded that of flight crews, or flight crews who participated in static training missions, probably had even higher exposures. UNDERESTIMATES RESULTING FROM ALL POSSIBLE PATHWAYS NOT BEING FACTORED INTO SOME GUIDELINES The committee used the existing indoor contamination guidelines for TCDD that were presented in Chapter 2 as a means of assessing the degree to which possible exposures of the AF Reservists may indicate possible adverse health consequences.
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From page 63... ...
Dermal absorption of vapor-phase TCDD is not considered in any of the proposed protocols. Given that some potentially important pathways are ignored, and that others are treated in a perfunctory manner, the degree of protectiveness of the existing guidelines expressed in terms of surface loading or air concentration should not be assumed.
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From page 64... ...
Compounds for which absorption of vapor via skin might plausibly exceed absorption via inhalation include both TCDD and the n-butyl esters of the herbicides 2,4-D and 2,4,5-T. PLAUSIBILITY OF AO-RELATED IMPAIRMENT OF HEALTH AMONG AF RESERVE PERSONNEL With increasing awareness of the toxic potential of various agents in the environment, substantial resources have been expended to develop methods to evaluate health risks in a rational and consistent fashion.
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From page 65... ...
Thus, the committee based its conclusions on a qualitative evaluation of the AF Reservists' potential exposures, although quantitative estimates based on various models and assumptions were explored. The committee considers the AF Reservists to have been exposed to TCDD and herbicide through multiple routes when on aircraft that had previously been used in ORH.
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From page 66... ...
Because of the methods used to collect the air samples, the committee did not consider the resulting measurements useful. Surface contamination with an SVOC can lead to exposure by inhalation of the chemical released into air, dermal absorption resulting from contacting a contaminated surface, and ingestion arising from hand-to-mouth transfer.
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From page 67... ...
Because of the sparse ature n of the available sampling results and the committee's instruction to perform a qualitative assessment of exposure, such an intricate approach was not deemed necessary or scientifically credible. The limitations of the available sampling data (as described previously)
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From page 68... ...
Therefore, in its efforts to establish the plausible magnitude of the AF Reservists' exposures, the committee considered values reflective of their actual work experience, rather than the extreme or "worst-case" values that are often used in risk assessments. When putting its perceptions of the available data in context, however, the committee accepted international screening guidelines, generally derived in accord with the precautionary principle, as defining a range of values at which taking further action in the interest of health would be merited.
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From page 69... ...
These arguments recognize considerable uncertainty, but they do provide support for concluding that the reported TCDD surface levels are very unlikely to be systematic overestimates in comparison to the existing surface loading guidelines. Table 4-4 is a compilation of sources of uncertainty demonstrating that there are a substantial number that might be expected to tend toward underestimation of the exposures and associated health risks experienced by the AF Reservists, which together may go far toward neutralizing the protective bias normally built into guidelines.
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From page 70... ...
discussed in this report (such as the long delay between when the activities leading to possible exposure occurred and sampling, the failure to adequately account for the extent of dermal absorption, considering only TCDD measurements in comparison to the screening levels developed in terms of TEQs for all dioxin-like chemicals contributing to exposure overall, etc.) would be expected to bias exposure estimates toward underestimation and hence an understatement of projected risk for the AF Reservists.
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From page 71... ...
TCDD (ng/m2) 100 10 1 0.1 1960 1970 1980 1990 2000 2010 { 1962–1971: ORH } { 1972–1982: Period of Post-Vietnam Use } Year FIGURE 4-1 Existing guidelines for TCDD Surface concentrations in relation to results of interior wipe samples.
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