critically and considered its relevance and quality. The committee did not collect original data, nor did it perform any secondary data analysis.
To be comprehensive in its approach to the epidemiologic literature, the committee also reviewed the studies that had been included in Volume 4 as primary or secondary studies. The Volume 4 committee did not draw conclusions about the strength of associations between specific exposures experienced during the Gulf War and particular health effects. However, Volume 4 did indicate what health outcomes were more prevalent in deployed veterans than in nondeployed veterans. The Update committee has been asked to determine the strength of associations between being deployed to the Gulf War and specific health effects. To make such a determination, the committee needed to review the studies cited in Volume 4 to assess whether they would still be classified as primary or secondary. The committee also tried to be comprehensive in its review of the literature inasmuch as the strength of an association would rest on a weight-of-the-evidence approach; that is, the committee considered it important to evaluate all primary and secondary studies that identified health outcomes so that a complete picture of the body of evidence could be presented.
The Update committee then considered the epidemiologic studies identified in the updated literature search. Those studies were also reviewed and classified as primary or secondary according to the criteria discussed below. Once the committee had assessed the studies cited in Volume 4 and evaluated the new studies identified in the more recent literature, it considered the entire body of relevant literature and determined the strength of associations between being deployed to the Gulf War and specific health outcomes on the basis of all the primary studies and supported by the secondary studies.
Because the committee was not attempting to link health outcomes to exposures other than deployment to the Persian Gulf theater, for which there is no known animal model, it did not review toxicologic, animal, or experimental studies comprehensively; however, it did evaluate the key epidemiologic and animal cited in the RAC report (see Appendix A). Epidemiologic studies that attempted to associate health effects in Gulf War veterans with specific exposures, such as oil-well fire smoke or nerve-gas agents, were also considered by the committee.
The Volume 4 committee cataloged the health outcomes that appeared to be more prevalent in veterans who had been deployed to the Gulf War than in veterans who served in the military at the same time but were not deployed to the Persian Gulf area, but it did not evaluate the strength of associations between deployment to the Gulf War and specific health outcomes. Studies were categorized as to whether the health outcomes seen in veterans were based on self-reports (including self-reports of physician diagnoses) or on objective measures, such as results of physical examinations by health-care providers or of laboratory tests. Using that approach, the Volume 4 committee found that on the basis of self-reports, deployed veterans had more symptoms indicative of multisymptom illness (although the symptoms did not appear to constitute a unique syndrome, illness, or symptom complex), such psychiatric disorders as posttraumatic stress disorder (PTSD), gastrointestinal disorders, skin disorders, joint pain, and respiratory disorders. However, when objective measures were used to diagnose the health outcomes seen in deployed and nondeployed veterans, different results were seen. Deployed veterans were more likely than nondeployed veterans to experience injury or death from