Limited/Suggestive Evidence of No Association

Evidence is consistent in not showing an association between exposure to a specific agent and a specific health outcome after exposure of any magnitude. A conclusion of no association is inevitably limited to the conditions, magnitudes of exposure, and length of observation in the available studies. The possibility of a very small increase in risk after exposure cannot be excluded.

The committee endeavored to express its judgment as clearly and precisely as the available data allowed, and it used the established categories of association from previous IOM studies because they have gained wide acceptance over more than a decade by Congress, government agencies, researchers, and veterans groups. The five categories describe different degrees of association and sound a recurring theme: the validity of an association is likely to vary with the extent to which the authors reduced common sources of error—chance variation, bias, and confounding—in drawing inferences. Accordingly, the criteria for each category express a degree of confidence based on the extent to which sources of error were reduced.


The committee reviewed numerous epidemiologic studies to arrive at conclusions about association. The committee weighed the strengths and limitations of all the epidemiologic studies and reached its conclusions by interpreting the data in the entire body of reviewed literature. It assigned each health outcome being considered to one of the five categories of association according to the criteria set forth above. The health outcomes that were indicated by the epidemiologic studies were numerous and include cancer, respiratory, cardiovascular, reproductive, and neurologic outcomes. In many cases, the health outcomes described might also result from genetics or lifestyle factors; for example, according to the American Cancer Society, smoking is believed to be responsible for about 80% of lung-cancer cases.

The committee’s findings about the strength of the associations between the putative agents and the health outcomes are summarized in Table ES-1.


Although Table ES-1 provides a summary of all the committee’s conclusions, the committee wishes to note that the starting point for any health outcome before study is, of course, the category of inadequate or insufficient evidence of an association. Of all the long-term health outcomes on which any evidence was culled from the epidemiologic literature, none was found to be associated in even a limited or suggestive fashion with the uncombusted fuels that veterans may have been exposed to during the Gulf War (with the exception of benzene, a component of fuels, which was reviewed by a previous IOM committee and not reassessed in this report).

The strongest finding was that there is sufficient evidence of an association between combustion products and lung cancer. The committee also found limited or suggestive evidence of an association between combustion-product exposure and cancers at several other sites (oral, nasal, laryngeal, and bladder), incident asthma, and two reproductive outcomes after exposure during pregnancy: preterm birth and low birthweight or intrauterine growth retardation. For the

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