scientific evidence and the appropriateness of the scientific methods used to detect the association,

  1. the increased risk of the illness among human or animal populations exposed to the agent, hazard, or medicine or vaccine,

  2. whether a plausible biologic mechanism or other evidence of a causal relationship exists between exposure to the agent, hazard, or medicine or vaccine and the illness.

It should be noted that the charge to IOM was not to determine whether a unique Gulf War syndrome exists or to make judgments about whether veterans were exposed to the putative agents. Nor was the charge to focus on broader issues, such as the potential costs of compensation for veterans or policy regarding compensation; such decisions are the responsibility of the secretary of veterans affairs.

Evidence of Statistical Association

The committee reviewed the available scientific evidence in the peer-reviewed literature to draw conclusions about associations between the agents of interest and adverse health effects. The committee placed its conclusions in categories that reflect the strength of the evidence of an association (described below). In an effort to determine whether a statistical association between a putative agent and a health outcome exists, the committee adapted categories of association used by the International Agency for Research on Cancer in evaluating evidence of the carcinogenicity of various agents and categories used by numerous other IOM committees.

Determining Increased Risk in Gulf War Veterans

The second part of the committee’s charge, as noted in the legislation, is to determine, to the extent permitted by available scientific data, the increased risk of illness among people exposed to the putative agents during service in the Persian Gulf. Generally, to accomplish that task, the committee would have reviewed studies of Gulf War veterans. However, many of the Gulf War veteran studies were hampered by poor measures of exposure to the putative agents, used questionnaires to identify illnesses and exposure to the agents of concern, or did not include outcomes measured with clinical examinations or laboratory tests. The committee therefore based its conclusions primarily on evidence from studies of people exposed to the putative agents in occupational or clinical settings rather than evidence from studies of Persian Gulf veterans. The committee found the evidence from occupational studies adequate for drawing conclusions about associations between the putative agents and health outcomes, but the lack of adequate data on the veterans themselves complicated its consideration of the second part of the charge: determination of increased risk in Gulf War veterans.

To estimate the magnitude of risk of a particular health outcome among Gulf War veterans, the committee would need to compare the rates of disease or other health effects in veterans exposed to the putative agents with the rates in those who were not exposed. That would require information about the specific agents to which individual veterans were exposed and about their doses. However, there is a paucity of data regarding the agents and doses to which individual Gulf War veterans were exposed. Furthermore, to answer questions about increased risk of illnesses in Gulf War veterans, it would be important to know the degree to which any other differences between exposed and nonexposed veterans could influence the rates of disease or other health outcomes; such information on the Gulf War veteran population is lacking.

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