the specificity of exposure assessment is an important consideration in weighing evidence relevant to the committee’s charge.
This chapter reviews the association between exposure to the chemicals of interest and neurobehavioral disorders, neurodegenerative disorders, and chronic peripheral system disorders. The scientific evidence supporting biologic plausibility is also reviewed here. More complete discussions of the categories of association and of this committee’s approach to categorizing health outcomes are presented in Chapters 1 and 2. For citations new to this update that revisit previously studied populations, design information can be found in Chapter 5.
This section summarizes the findings of VAO and previous updates on neurobehavioral disorders and incorporates information published in the last 2 years into the evidence database.
Conclusions from VAO and Previous Updates
On the basis of the data available at the time, the committees responsible for VAO, Update 1996, Update 1998, Update 2000, Update 2002, Update 2004, Update 2006, and Update 2008 concluded that there was inadequate or insufficient evidence to determine whether there is an association between exposure to the chemicals of interest and neurobehavioral disorders. Many of the data that informed that conclusion came from the Air Force Health Study (AFHS, 1991, 1995, 2000; Barrett et al., 2001, 2003). VAO and the updates offer more complete discussions of the results. The AFHS studies (AFHS, 1991, 1995) reviewed in VAO revealed no association between serum TCDD concentration and reported sleep disturbance or variables on the Symptom Checklist-90-Revised (SCL-90); in contrast, serum TCDD was significantly associated with responses on some scales of the Millon Clinical Multiaxial Inventory. Observations on 55 highly exposed Czech 2,4,5-T production workers (Pazderova-Vejlupkova et al., 1981) were found to suffer from methodologic problems.
Update 1996 reviewed two not particularly informative studies of Vietnam veterans (Decoufle et al., 1992; Visintainer et al., 1995) and a study of highly exposed German workers (Zober et al., 1994), which found a relationship between “mental disorders” and severity of chloracne but not with blood TCDD concentrations. Update 1998 considered a report on mental-health problems in Australian Vietnam veterans but not in the context of herbicide exposure (O’Toole et al., 1996).
In Update 2000, results from the AFHS (AFHS, 2000) indicated that although the frequency of several self-reported neuropsychiatric symptoms differed between exposure groups, the associations were not significant after adjustment