MILITARY USE OF HERBICIDES IN VIETNAM

Military use of herbicides in Vietnam took place from 1962 through 1971. Tests conducted in the United States and elsewhere designed to evaluate defoliation efficacy were used to select specific herbicides (IOM, 1994; Young and Newton, 2004). Four compounds were used in the herbicide formulations in Vietnam: 2,4-dichlorophenoxyacetic acid (2,4-D), 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), 4-amino-3,5,6-trichloropicolinic acid (picloram), and dimethylarsinic acid (cacodylic acid). The chemical structures of those compounds are presented in Chapter 2 (see Figure 2-1). The herbicides were used to defoliate inland hardwood forests, coastal mangrove forests, cultivated lands, and zones around military bases. In 1974, a National Resource Council committee estimated the amount of herbicides sprayed from helicopters and other aircraft by using records gathered from August 1965 through February 1971 (NRC, 1974). That committee calculated that about 18 million gallons (about 69 million liters) of herbicide was sprayed over about 3.6 million acres (about 1.5 million hectares) in Vietnam in that period. The amount of herbicides sprayed on the ground to defoliate the perimeters of base camps and fire bases and the amount sprayed by Navy boats along riverbanks were not estimated.

A revised analysis of spray activities and exposure potential of troops emerged from a study overseen by a committee of the Institute of Medicine (IOM, 1997, 2003a,b). That work yielded new estimates of the amounts of military herbicides used in Vietnam from 1961 through 1971 (Stellman et al., 2003a). The investigators reanalyzed the original data sources that were used to develop herbicide-use estimates in the 1970s and identified errors that inappropriately removed spraying missions from the dataset. They also added new data on spraying missions that took place before 1965. Finally, a comparison of procurement records with spraying records found errors that suggested that additional spraying had taken place but gone unrecorded at the time. The new analyses led to revision of estimates of the amounts of the agents applied, as indicated in Table 3-1. The new research effort estimated that about 77 million liters were applied, about 9 million liters more than the previous estimate.

Table 3-1

Military Use of Herbicides in Vietnam (1961–1971).

Herbicides were identified by the color of a band on 55-gallon shipping containers and were called Agent Pink, Agent Green, Agent Purple, Agent Orange, Agent White, and Agent Blue. Agent Green and Agent Pink were used in 1961 and 1965, and Agent Purple in 1962–1965. Agent Orange was used in 1965–1970, and a slightly different formulation (Agent Orange II) probably was used after 1968. Agent White was used in 1966–1971. Agent Blue was used in powder form in 1962–1964 and as a liquid in 1964–1971. Agent Pink, Agent Green, Agent Purple, Agent Orange, and Agent Orange II all contained 2,4,5-T and were contaminated to some extent with TCDD. Agent White contained 2,4-D and picloram. Agent Blue (powder and liquid) contained cacodylic acid. The chlorinated phenoxy acids 2,4-D and 2,4,5-T persist in soil for only a few weeks; picloram is much more stable, persisting in soil for years; and cacodylic acid is nonvolatile and stable in sunlight (NRC, 1974). More details on the herbicides used are presented in the initial IOM report, Veterans and Agent Orange: Health Effects of Herbicides Used in Vietnam, referred to as VAO (IOM, 1994).

TCDD IN HERBICIDES USED IN VIETNAM

TCDD is formed during the manufacture of 2,4,5-T in the following manner: trichlorophenol (2,4,5-TCP), the precursor for its synthesis, is formed by the reaction of tetrachlorobenzene and sodium hydroxide (see Figure 3-1a); 2,4,5-T is formed when 2,4,5-TCP reacts with chloroacetic acid (see Figure 3-1b); small amounts of TCDD are formed as a byproduct of the intended main reaction (see Figure 3-1b) when a molecule of 2,4,5-TCP reacts with the tetrachlorobenzene stock (see Figure 3-1c) instead of with chloroacetic acid. In each step in the reaction, a chlorine atom is replaced with an oxygen atom, and this leads to the final TCDD molecule (NRC, 1974). In the class of compounds known as polychlorinated dibenzo-p-dioxins (PCDDs), 75 congeners can occur, depending on the number and placement of the chlorine atoms. Cochrane et al. (1982) noted that TCDD had been found in pre-1970 samples of 2,4,5-TCP. Other PCDDs—2,7-dichloro-dibenzo-p-dioxin and 1,3,6,8-tetrachloro-dibenzo-p-dioxin—were measured in the same samples. The concentration of TCDD in any given lot of 2,4,5-T depended on the manufacturing process (FAO/UNEP, 2009; Young et al., 1976).

FIGURE 3-1

TCDD formation during 2,4,5-T production.

The manufacture of 2,4-D is a different process: its synthesis is based on dichlorophenol, a molecule formed from the reaction of phenol with chlorine (NZIC, 2009). Neither tetrachlorobenzene nor trichlorophenol is formed during this reaction, so TCDD is not normally a byproduct of the manufacturing process. However, other, less toxic PCDDs have been detected in pre-1970 commercial-grade 2,4-D (Cochrane et al., 1982; Rappe et al., 1978; Tosine, 1983). Cochrane et al. (1982) found multiple PCDDs in isooctyl ester, mixed butyl ester, and dimethylamine salt samples of 2,4-D. It has also been noted that cross-contamination of 2,4-D with 2,3,7,8-TCDD occurred in the operations of at least one major manufacturer (Lilienfeld and Gallo, 1989).

TCDD concentrations in individual herbicide shipments were not recorded but were known to vary from batch to batch and between manufacturers. TCDD concentrations in stocks of Agent Orange remaining after the conflict, which either had been returned from South Vietnam or had been procured but not shipped, ranged from less than 0.05 ppm to almost 50 ppm and averaged 2–3 ppm in two sets of samples (NRC, 1974; Young et al., 1978). Comparable manufacturing standards for the domestic use of 2,4,5-T in 1974 required that TCDD not be present at over 0.05 ppm (NRC, 1974).

Data from Young and Gough were originally used to estimate the amount of TCDD in the various herbicide formulations (Gough, 1986; Young, 1992; Young et al., 1978). Young et al. (1978) estimated that Agent Green, Agent Pink, and Agent Purple—used early in the program (through 1965)—contained 16 times the mean TCDD content of the formulations used in 1965–1970, and mean TCDD concentrations in Agent Pink and Agent Green were estimated at 66 ppm. Gough (1986) estimated that about 167 kg of TCDD was sprayed in Vietnam over a 6-year period.

Later analysis by researchers at Columbia University benefited from access to military spray records that had not been available earlier and has resulted in substantial revisions of the estimates (Stellman et al., 2003a). The investigators were able to incorporate newly found data on spraying in the early period of the war (1961–1965) and to document that larger volumes of TCDD-containing herbicides were used in Vietnam than had been estimated previously. They also found the earlier estimates of TCDD contamination in the herbicide formulations to be low, noting that the original estimates were based on samples at the lower end of the distribution of concentration. They concluded that the mean TCDD concentration in Agent Orange was closer to 13 ppm than to the earlier estimate of 3 ppm. They therefore proposed 366 kg of TCDD as a plausible estimate of the total amount of TCDD applied in Vietnam during 1961–1971.

EXPOSURE OF VIETNAM VETERANS

Determination of exposures of US military personnel who served in Vietnam has been perhaps the greatest challenge in the study of health effects associated with herbicides and TCDD. Some military personnel stationed in cities or on large bases may have received little or no herbicide exposure, whereas troops who moved through defoliated areas soon after treatment may have been exposed through soil contact, drinking water, or bathing. Reliable estimates of the magnitude and duration of such exposures are not possible in most cases, given the lack of contemporaneous chemical measurements, the lack of a full understanding of the movement and behavior of the defoliants in the environment, and the lack of records of individual behaviors and locations. Consequently, most studies have focused on populations that had well-defined tasks that brought them into contact with the agents. It is believed that the subjects of those studies, primarily Air Force personnel involved in fixed-wing aircraft spraying activities (often referred to as Operation Ranch Hand) and members of the US Army Chemical Corps (ACC), may have had among the highest exposures. As described below, exposures of ground troops are difficult to define, so this group has not been studied as intensively. In accord with Congress's mandated presumption of herbicide exposure of all Vietnam veterans, VAO committees have treated Vietnam-veteran status as a proxy for some herbicide exposure when more specific exposure information is not available.

EXPOSURE OF THE VIETNAMESE POPULATION

As summarized by Constable and Hatch (1985), Vietnamese researchers have made a number of attempts to characterize the herbicide exposure of residents of Vietnam in the process of trying to assess adverse reproductive outcomes. Some compared residents of the South with residents of the unsprayed North, and others endeavored to compare South Vietnamese people who lived in sprayed and unsprayed villages as determined by observed defoliation. For evaluating reproductive outcomes, pregnancy outcomes of North Vietnamese women married to veterans who had served in South Vietnam were compared with those of women whose husbands had not. In some cases, records of herbicide spraying have been used to refine exposure measurements. In assessing infant mortality, Dai et al. (1990) considered village residents to have been exposed if a herbicide mission had passed within 10 km of the village center and classified exposure further by length of residence in a sprayed area and the number of times that the area reportedly had been sprayed.

A small number of studies have provided information on TCDD concentrations in Vietnamese civilians who were exposed during the war (Schecter et al., 1986, 2002, 2006). Dwernychuk et al. (2002) emphasized the need to evaluate dioxin contamination around former air bases in Vietnam. They collected environmental and food samples, human blood, and breast milk from residents of the Aluoi Valley of central Vietnam. The investigators identified locations where relatively high dioxin concentrations remained in soil or water systems. Soil dioxin concentrations were particularly high around former airfields and military bases where herbicides were handled. Fish harvested from ponds in those areas were found to contain high dioxin concentrations. More recently, Dwernychuk (2005) elaborated on the importance of “hot spots” as important locations for future studies and argued that herbicide use at former US military installations was the most likely cause of the hot spots. The Bien Hoa Air Base, considered a hot spot because of the use of chemical defoliants around the base, was the focus of a study that examined dioxin contamination in soils in Vietnam (Mai et al., 2007). The study found high soil concentrations but did not involve estimation of the exposure of people who lived in the vicinity of the bases.

Several publications have reported environmental concentrations and body burdens of dioxins in various areas throughout Vietnam (Brodsky et al., 2009; Feshin et al., 2008; Hatfield Consultants, 2009a,b,c; Nhu et al., 2009; Saito et al., 2010; Tai et al., 2011). Like previous publications, that by Tai et al. (2011) reported that dioxin concentrations in breast milk were related to the residence location of the mothers, with levels and total toxic equivalents (TEQs) highest in areas where herbicides were stored and sprayed during the war. PCDD and PCDF TEQ, for example, were about three times higher in the sprayed areas and four times higher in the hot spots than in unsprayed areas. However, dioxin concentrations in breast milk, even in women who lived in sprayed areas or hot spots, were lower than those documented in previous publications. The differences were attributed to the fact that the study characterized dioxin in the general population compared with the highly exposed populations of previous studies. Nevertheless, the findings of Tai et al. (2011) were consistent with earlier findings that showed pervasive exposure to dioxins more than a half-century after the Vietnam War.

Additional studies have addressed dioxin uptake in other contaminated environments. For example, Tohyama et al. (2011) assessed the exposure of 138 people, including 66 children 3–15 years old, who lived on dioxin-contaminated soil (concentrations up to 6,800 ppt) in Tokyo, Japan, focusing on people who were suspected of being exposed to the contaminated soil. For those at least 16 years old, the blood dioxin concentrations did not vary with the estimated level of contamination and were higher (10 ± 0.54 pg TEQ/g lipid) than those observed in American Vietnam-era veterans, which averaged 4 ppt in both the deployed and the nondeployed groups (CDC, 1988). For the children, however, the mean concentrations were 13 (± 1.9) and 6.6 (± 0.65) pg TEQ/g lipid (equivalent to ppt) for the 3- to 6-year-olds and the 7- to 15-year-olds, respectively, and those who were fed only breast milk had higher concentrations than did those fed a combination of breast milk and formula or only formula. The range observed in the youngest age group (0.94–46 pg TEQ/g lipid) bracketed the readings for all 138 subjects.

Demond et al. (2012) noted the results of the University of Michigan Dioxin Exposure Study of 946 residents of the countries surrounding the Dow facilities (Demond et al., 2008, Garabrant et al., 2009) and reviewed other studies that measured concentrations in human serum arising from exposure to dioxin-contaminated soil. Their overall conclusion was that serum TCDD concentrations did not directly reflect the degree of soil contamination; consumption of animal products raised on the contaminated soil, however, was related to increases in serum TEQs.

The above studies are not directly relevant to the present committee's task, but they may prove useful in future epidemiologic studies of the Vietnamese population and in the development of risk-mitigation policies.

MODELS FOR CHARACTERIZING HERBICIDE EXPOSURE¹

The IOM, following up on the recommendations contained in the original VAO report (IOM, 1994), issued a request for proposals seeking individuals and organizations to develop historical exposure-reconstruction approaches suitable for epidemiologic studies of herbicide exposure of US veterans during the Vietnam War (IOM, 1997). The request resulted in the project Characterizing Exposure of Veterans to Agent Orange and Other Herbicides in Vietnam. The project was carried out under contract by a team of researchers in Columbia University's Mailman School of Public Health. The Columbia University project integrated various sources of information concerning spray activities and information on locations of military units assigned to Vietnam, all compiled into a database, to generate individualized estimates of the exposure potential of troops serving in Vietnam (Stellman and Stellman, 2003)

“Mobility-factor” analysis, a new concept for studying troop movement, was developed for use in reconstructing herbicide-exposure histories. The analysis is a three-part classification system for characterizing the location and movement of military units in Vietnam. It comprises a mobility designation (stable or mobile), a distance designation (usually in kilometers) to indicate how far a unit might travel in a day, and a notation of the modes of travel available to the unit (by air, by water, or on the ground by truck, tank, or armored personnel carrier). A mobility factor was assigned to every unit that served in Vietnam.

The data were combined into a geographic information system (GIS) for Vietnam. Herbicide-spraying records were integrated into the GIS and linked with data on military-unit locations to derive individual exposure-opportunity scores. The results are the subject of reports by the contractor (Stellman and Stellman, 2003) and the Committee on the Assessment of Wartime Exposure to Herbicides in Vietnam (IOM, 2003a,b). A summary of the findings on the extent and pattern of herbicide spraying (Stellman et al., 2003a), a description of the GIS for characterizing exposure to Agent Orange and other herbicides in Vietnam (Stellman et al., 2003b), and an explanation of the exposure-opportunity models based on that work (Stellman and Stellman, 2004) have been published in peer-reviewed journals. The publications have argued that it is feasible to conduct epidemiologic investigations of veterans who served as ground troops during the Vietnam War. The IOM later issued a report that examined the feasibility of using the Agent Orange Reconstruction Model developed by Columbia University (IOM, 2008). The report concluded that “despite the shortcomings of the exposure assessment model in its current form and the inherent limitations in the approach, the committee agreed that the model holds promise for supporting informative epidemiologic studies of herbicides and health among Vietnam veterans and that it should be used to conduct studies.”

A pair of industry-sponsored papers that used a mathematical model of herbicide dispersion and deposition from aerial spraying concluded that actual ground deposition of Agent Orange was many orders of magnitude lower than that predicted by previous exposure estimations proposed for use in evaluating ground-troop health effects (Ginevan et al., 2009a,b). The new papers first undertook a quantitative evaluation of the Stellman EOI model (Stellman and Stellman, 2004; Stellman et al., 2003a,b) recommended for possible use in an epidemiologic evaluation of ground troops by IOM (2008). The new evaluation revealed frequent and substantial inconsistencies in the calculated EOI that were based in part on the use of a central equation “contrary to a large body of pesticide exposure assessment practice,” the general imprecision of spray-flight path records, the use of 1.2-km² exposure cells in the model, and “unknown computational errors” in the model. The analyses demonstrated unexpected and unexplained 1,000-fold differences in model output for sample flight paths that appear to be in all respects equivalent. The authors proposed the use of the AgDRIFT Tier III model as a more accurate and appropriate estimator of potential exposures for ground troops. That model uses a combination of standard Lagrangian and Gaussian techniques in combination with empirically derived information, such as aerosol penetration through a forest canopy, to estimate ground-level exposure. The AgDRIFT Tier III model is purportedly validated and used by the US Forest Service to plan aerial application of various agents to forests. The AgDRIFT model predicts a much smaller area under the spray path and Agent Orange concentrations lower by several orders of magnitude than the EOI estimates for the same set of sample flight paths. That effect is particularly pronounced at points distant from the spray path; the AgDRIFT model predicts Agent Orange exposures up to 20 orders of magnitude lower than the EOI model at a point 4 km away from the flight-path centerline. Finally, the authors pointed out that the use of any exposure model for ground troops will be severely limited by the imprecision of spatial and temporal measures of troop movements.

Stellman and Stellman (2013) provided the committee with a letter submitted to the editor of the Journal of Exposure Science and Environmental Epidemiology in response to the critique of their model in Ginevan et al. (2009b); they also provided supplemental material. In their letter, Stellman and Stellman, the senior authors of the EOI model, question the validity of most, if not all, of the Ginevan et al. (2009a,b) findings, citing serious errors regarding the paper's use of “incorrect data and its fundamentally incorrect and negative interpretations.” The Stellmans, for example, noted several key errors in Ginevan's reporting of “E4 scores,” which were used as the basis of much of the Ginevan et al. critique of the EOI model. Stellman and Stellman found that Ginevan et al. used in their analysis raw E4 scores, instead of the log-transformed E4 scores as used by the Stellmans. As a result, variability in the EOI on the flight line as reported by Ginevan et al. was artificially high; the Stellmans' reports that a log-transform of the E4 scores produces reasonable values that vary within l0% around the mean for each mission. Furthermore, the Stellmans' state that use of raw E4 scores leads to a host of other incorrect assertions and theories, such as “their use of an incorrect score of 60,79I for one point, when the true score is zero in our [the EOI] system.”

Given the lack of data to validate either model and the lack of an impartial evaluation of both models by a third party, it is not possible for the committee to ascertain the accuracy and precision of estimates from either model or the claims of either Stellman and Stellman (2013) or Ginevan et al. (2009a,b). It should also be noted that because the intent and outcomes of the two models differ substantially, model results and interpretation are likely to differ. The Stellman and Stellman model, for example, predicts potential exposure to troops on the basis of military data on spray history and troop locations. The AgDRIFT Tier III model, in contrast, predicts ground concentrations and their spatial dispersion; by design, the AgDRIFT model does not consider troop-location data. Many studies have shown that the agreement and correlation between pollutant exposures and concentrations can be poor, so it is not surprising that the results and interpretation of two models differ substantially, especially given command directives that prohibited spraying when Allied troops were on the ground. The command directives suggest that the opportunity for Vietnam veterans to be exposed would be lowest in areas under the spray path, where concentrations would be highest.

Since Update 2010, Young and Cecil (2011) have published a review that states that few, if any, ground-troop veterans were exposed to Agent Orange on the basis of arguments made earlier (Young et al., 2004a,b). They also state that the EOI model is flawed, given deficiencies in model assumptions concerning spray operations and areas; information not considered by the model, such as meteorologic characteristics; and command directives that prohibited spraying when Allied troops were on the ground in the areas to be sprayed.

The issue of Allied troop presence during spraying is one of the central issues in the debate regarding the use of the EOI model. The EOI model relied on actual military data on spray history and troop locations, which, as pointed out both by Stellman and Stellman (2004) and Young (2009), are limited in their spatial and temporal resolution and accuracy. The accuracy of the records with regard to missions flown, mission locations, number of gallons sprayed, among other important information, was examined by MITRE Corporation (Heizer, 1971). MITRE reported that about 2% of the records were missing data, 6% of the records had serious transcription or measurement errors, and 23% of records that had complete data were off by 50% in the reported distance sprayed (Young, 2009). However, the overall quality of the data was found to be good and could be improved with adjustments, as performed by Stellman (2003a,b) and others (ESG, 1985; NRC, 1974). Whether the adjustments improved the quality of the military data is not known, but Young (2009) has criticized methods used to adjust the records.

Nevertheless, the committee cannot dismiss EOI model findings solely on the basis that command directives prohibited spraying, given that the EOI model is based on actual military data. Inasmuch as the AgDRIFT and EOI models focus on different outcomes, however, the committee does not recommend that one model be used instead of the other for the purposes of epidemiologic studies, nor does it advocate or discourage use of either the AgDRIFT or the EOI model in epidemiologic studies. If either model is used in epidemiologic studies to predict exposure, results should be interpreted in light of the model limitations noted.

The controversy surrounding the use of the EOI and AgDRIFT models points to the difficulties inherent in assessing Agent Orange exposures of Vietnam veterans. For Operation Ranch Hand and ACC cohorts, exposure assessment is the most straightforward of all assessments of Vietnam veterans in that their exposures to Agent Orange originate predominantly from one source and one exposure route. Nevertheless, attempts to quantify their exposures, even at the level of serum biomarkers of exposure, have been less than satisfactory. For ground troops, who make up the largest group of concern, exposure assessment is considerably more complex; multiple, dispersed sources of Agent Orange exposure over multiple possible routes occurred over an extended period long ago. As a result, few studies have characterized exposure beyond “in-theater” vs “not-in-theater” comparisons. Considerable work, however, has been done by National Academies committees and others to develop exposure assessments for ground troops based on numbers, patterns, and timing of aerial spray missions combined with troop-location information. Aerial spraying has been the focus of much of the committee's efforts given that 95% of Agent Orange used during the Vietnam War was applied by aerial spraying (Stellman et al., 2003a,b).

Regardless, it is important to note that sole emphasis on aerial spraying as an exposure source should be reconsidered. To ascribe a health effect to an exposure in an epidemiologic study accurately, one must account for all sources and routes of exposure—a concept now popularly termed total exposure assessment. In the Vietnam theater, there were undoubtedly multiple sources and routes of TCDD exposure of ground troops other than being directly under an aerial-spray mission. The relative magnitudes of those sources and whether the aerial spray route predominated are unknown and now probably unknowable. For instance, troops in the field commonly collected drinking water from streams. Some of those streams are still highly polluted with TCDD. Although the ultimate source of the TCDD in the streams may have been aerial spraying, the concentration of TCDD in the water would not necessarily correlate with spray-mission exposure estimates and could conceivably far exceed the “direct exposure” estimates, depending on the terrain, rainfall, timing of water collection, and other unknown factors. The dynamic nature of TCDD released into the environment is largely unknown quantitatively, so an exposure assessment that accounts for all sources of TCDD exposure is impossible. In addition, an assessment of total exposure must include an understanding of coexposures that could confound TCDD exposure analyses or otherwise directly account for an observed health effect. Studies have not factored coexposures into health risk estimates.

METHODOLOGIC ISSUES IN EXPOSURE ASSESSMENT

Analyses of Vietnam-veteran studies have been an important source of information for understanding associations between the herbicides used in Vietnam and specific health outcomes, but, as discussed in Chapter 2, the committee has extended its review of the scientific literature to other populations whose exposure could be estimated with greater accuracy. Those populations are discussed in detail in Chapter 5. We focus here on several key methodologic issues that complicate development of accurate estimates of exposure of the Vietnam-veteran population and the other study populations discussed in this report: the latent period between exposure and disease, exposure misclassification, and exposure specificity.

Specificity

Incorporation of findings of studies of persons exposed to components of the herbicides sprayed in Vietnam requires some decisions about their relative contributions to the VAO project's evidentiary database. Only a few herbicidal chemicals were used as defoliants during the Vietnam conflict: esters and salts of 2,4-D and 2,4,5-T, cacodylic acid, and picloram in various formulations. Many scientific studies reviewed by the committee report exposures to broad categories of chemicals rather than to those specific chemicals. The categories are presented in Tables 3-2 and 3-3 with their relevance to the committee's charge. The information in these tables has helped to guide the committee's evaluation of epidemiologic studies. Earlier VAO committees did not address the issue of exposure specificity in exactly this manner. The committee for VAO and the first several updates gave more weight to results that were based on job title (for example, “farmer” with no additional information) than have the committees for the last four updates, but they entirely excluded findings from the Yusho and Yucheng polychlorinated dibenzofuran and biphenyl (PCDF and PCB) poisonings, whereas recent committees have considered studies that analyzed for dioxin-like PCDF and PCB congeners and expressed the results in terms of TEQs. Nonetheless, those studies that report TEQs based only on mono-ortho PCBs (which are PCBs 105, 114, 118, 123, 156, 157, 167, and 189) were given very limited consideration since mono-ortho PCBs typically contribute less than 10% to total TEQs, based on the World Health Organization revised TEFs of 2005 (La Rocca et al., 2008; van den Berg et al., 2006).

TABLE 3-2

Current Committee Guidance for the Classification of Exposure Information in Epidemiologic Studies That Focus on the Use of Pesticides or Herbicides, and Relevance of the Information to the Committee's Charge to Evaluate Exposures to 2,4-D and 2,4,5-T (more...)

TABLE 3-3

Current Committee Guidance for the Classification of Exposure Information in Epidemiologic Studies That Focus on Exposure to Dioxin-Like Chemicals and Relevance of the Information to the Committee's Charge.

Many studies have examined the relationship between exposure to “pesticides” and adverse health outcomes, and others have used the category of “herbicides” without identifying specific chemicals. A careful reading of a scientific report often reveals that none of the chemicals of interest (COIs) (that is, those used in Vietnam, as delineated above) contributed to the exposures of the study population, so such studies could be excluded from consideration. But in many cases, the situation is more ambiguous. For example, reports that define exposure in the broad category of “pesticides,” with no further information, have little relevance to the committee's charge to determine associations between exposures to herbicides used in Vietnam and adverse health outcomes. Reports that define exposure in the more restricted category of “herbicides” are of greater relevance but are of little value unless it is clear from additional information that exposure to one or more of the herbicides used in Vietnam occurred in the study population—for example, if a published report indicates that the COIs were among the pesticides or herbicides used by the study population, the lead author of the report has been contacted and has indicated that the COIs were among the chemicals used, the COIs are used commonly for the crops identified in the study, or the COIs are used commonly for a specific purpose, such as removal of weeds and shrubs along highways.

Among the various chemical classes of herbicides that have been identified in published studies reviewed by the committee, phenoxy herbicides, particularly 2,4-D and 2,4,5-T, are directly relevant to the exposures experienced by US military forces in Vietnam. On the basis of the assumption that compounds with similar chemical structure may have analogous biologic activity, information on the effects of other chemicals in the phenoxy herbicide class—such as Silvex, 2-methyl-4-chlorophenoxyacetic acid, 2-(2-methyl-4-chlorophenoxy) propionic acid (Mecoprop), and dicamba—has been factored into the committee's deliberations with somewhat less weight. The very few epidemiologic findings on exposure to picloram or cacodylic acid have been regarded as highly relevant. The committee has decided to include many studies that report on unspecified herbicides in the health-effects sections, and their results have been entered into the health-outcome–specific tables; however, these studies tend to contribute little to the evidence considered by the committee. The many studies that provide chemical-specific exposure information are believed to be far more informative for the committee's purposes.

A similar issue arises in the evaluation of studies that document exposure to dioxin-like compounds. Most “dioxin” studies reviewed by the committee have focused on TCDD, but TCDD is only one of a number of PCDDs. The committee recognizes that in real-world conditions exposure to TCDD virtually never occurs in isolation and that there are hundreds of similar compounds to which humans might be exposed, including other PCDDs, polychlorinated dibenzofurans (PCDFs), and PCBs. Exposure to TCDD is almost always accompanied by exposure to one or more of the other compounds. The literature on the other compounds, particularly PCBs, has not been reviewed systematically by the committee unless TCDD was identified as an important component of the exposure or the risks of health effects were expressed in terms of TEQs, which are the sums of toxicity equivalence factors for individual dioxin-like compounds as measured by activity with the aryl hydrocarbon receptor (AHR). The committee took that approach for two reasons. First, exposure of Vietnam veterans to substantial amounts of the other chemicals, relative to exposure to TCDD, has not been documented. Second, the most important mechanism for TCDD toxicity involves its ability to bind to and activate the AHR. Many of the other chemicals act by different or multiple mechanisms, so it is difficult to attribute toxic effects after such exposures specifically to TCDD. Furthermore, people's environmental exposures to dioxin-like chemicals and their non–dioxin-like counterparts are to mixtures of components that tend to correlate, so it is not surprising that specific chemicals measured in a person's serum also tend to correlate; this means that it will be difficult for epidemiologic studies to attribute any observed association to a particular chemical configuration (Longnecker and Michalek, 2000). Analyses in terms of TEQs circumvent that problem to some extent.

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