(most importantly cohort, case–control, and cross-sectional) have strengths and weaknesses (see Chapter 2) that influence their potential to contribute evidence of an association with the health outcomes considered in Chapters 6–9 of this report. Update 2006 has retained the categorization scheme; cycling through these categories, the second part of this chapter discusses the design details of new reports on populations already under study and of studies on new populations reporting multiple endpoints. The occupational section covers studies of production workers, agriculture and forestry workers (including herbicide and pesticide applicators), and paper and pulp workers. The environmental section covers studies of populations unintentionally exposed to unusually high concentrations of herbicides or dioxins as a result of where they lived, such as Seveso, Italy; Times Beach, Missouri; and the southern portion of Vietnam. The section on Vietnam veterans covers studies of US veterans conducted by the Air Force, the Centers for Disease Control and Prevention (CDC), the Department of Veterans Affairs (VA), the American Legion, and individual states; it also discusses studies of veterans from other nations (e.g., Australia and Korea) that fought in Vietnam.
In addition to reviewing studies involving exposures to the chemicals of interest (2,4-D; 2,4,5-T and its contaminant TCDD; cacodylic acid; and picloram), this and earlier VAO committees have examined any available studies that address compounds chemically related to the herbicides used in Vietnam, such as 2-(2-methyl-4-chlorophenoxy) propionic acid (MCPP), hexachlorophene, and chlorophenols, particularly 2,4,5-trichlorophenol (2,4,5-TCP). Some study investigators do not indicate in their published reports the specific herbicides to which study participants were exposed or the magnitude of exposure; those complicating factors were considered when the committee weighed the relevance of a study, as detailed in Chapter 2 of this report. Available details of exposure assessment and use of the resulting data in analyses are discussed in Chapter 5, which follows the same sequence to categorize study populations.
To elucidate further the new epidemiologic data reviewed by the committee for Update 2006, three tables have been added to this chapter that list new citations. Tables 4-1 and 4-2 both list citations for studies of populations that have not been reviewed in previous updates; studies listed in Table 4-1 address single health outcomes, and studies in Table 4-2 address multiple health outcomes. Studies listed in Table 4-1 are discussed and critiqued in the health-outcome section for the outcome investigated. To reduce repetition in the report, studies in Table 4-2 are described in detail in this chapter and only briefly in the health-outcome sections; this avoids recapitulation of the information every time a new health outcome from the same study is discussed. Finally, new studies that report on populations that have been discussed in previous updates are listed in Table 4-3; these are described in detail in this chapter in the context of associated studies