humans, but is limited because chance, bias, and confounding could not be ruled out with confidence.

  • Inadequate/Insufficient Evidence to Determine Whether an Association Does or Does Not Exist. The available studies are of insufficient quality, consistency, or statistical power to permit a conclusion regarding the presence or absence of an association between an exposure to a specific agent and a health outcome in humans.

  • Limited/Suggestive Evidence of No Association. There are several adequate studies covering the full range of levels of exposure that humans are known to encounter that are mutually consistent in not showing a positive association between exposure to a specific agent and a health outcome at any level of exposure. A conclusion of no association is inevitably limited to the conditions, levels of exposure, and length of observation covered by the available studies. In addition, the possibility of a very small elevation in risk at the levels of exposure studied can never be excluded.

These five categories describe different strengths of association, with the highest level being sufficient evidence of a causal relationship between exposure to a specific agent and a health outcome. The criteria for each category sound a recurring theme: An association is more likely to be valid to the extent that the authors reduced common sources of error in making inferences—chance variation, bias in forming a study cohort, and confounding. Accordingly, the criteria for each category express varying degrees of confidence based upon the extent to which it has been possible to exclude these sources of error. To infer a causal relationship from a body of observational evidence, the committee relied on long-accepted criteria for assessing causation in epidemiology (Hill, 1971; Evans, 1976). The following sections provide a discussion and conclusions regarding the putative agents (DU, PB, sarin, and vaccines).

DEPLETED URANIUM

Depleted uranium is a by-product of the enrichment process used to make reactor-grade uranium. Natural uranium is considered a low-level radioactive element. Because of the different percentages of uranium isotopes, the specific activity (a measure of radioactivity) of depleted uranium (14.8 mBq/μg) is 40 percent lower than that of naturally occurring uranium (25.4 mBq/μg) and considerably lower than that of enriched uranium (approximately 1,750 mBq/μg) (Harley et al., 1999). However, the chemical properties of depleted uranium are the same as those of the enriched and naturally occurring forms.

The U.S. military used depleted uranium in the Gulf War for offensive and defensive purposes (OSAGWI, 1998). Heavy armor tanks had a layer of depleted uranium armor to increase protection. Depleted uranium was also used in kinetic energy cartridges and ammunition rounds. U.S. personnel were exposed to depleted uranium as the result of friendly fire incidents, cleanup operations, and



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