at the population level rather than at the individual level) cannot provide strong evidence of a link between burn pit exposure and a specific disease, but they may be useful for detecting possible increases of disease in groups generally assumed to be exposed (such as having ever been deployed to a site with an operating burn pit).
Identification of Study Populations
The selection of appropriate study and control groups is an essential step in the design of a study of military personnel exposed to burn pit emissions. The study sample should be representative of the population of interest and large enough to ensure adequate statistical power. In this case, the population of interest is military personnel who were stationed at JBB during the operation of the burn pit (2003–2009). The committee was provided with information from the Defense Manpower Data Center (DMDC) that indicated the U.S. military population within 10 miles of JBB increased from about 240 in 2003 to about 15,000 in 2007, and then it decreased to about 10,000 in 2009 (Steve Halko, Defense Manpower Data Center, personal communication, August 25, 2010). According to DMDC, the population at JBB might have reached 25,000 when coalition and host-nation forces, civilians, and contractors were included. Many of those stationed at JBB remained on the base for the duration of their tour of duty. With the cooperation of the DMDC, specific information on number of personnel deployed to various locations in or near JBB, the dates and number of their deployments, and other relevant deployment information could be obtained and used to identify an appropriate study cohort.
Military personnel are usually not comparable to the general U.S. population because of a number of factors, such as age and sex distributions, patterns of activity, and behaviors such as smoking, that make the selection of an appropriate control group particularly important and problematic. Although comparisons with the general population are not helpful, comparison within military populations suffer similar issues. For example, deployed military personnel may be healthier in terms of medical history and levels of fitness than nondeployed personnel, a “healthy warrior effect” that may operate even within the military (see Chapter 6). Furthermore, deployed personnel are also exposed to a variety of environmental and personal conditions that the general population and nondeployed personnel may never encounter (IOM 2010). Deployed and nondeployed military personnel may also experience significantly more stressors than the general population. According to a previous IOM report (IOM 2008a) “exposure to combat has been described as one of the most intense stressors that a person can experience; for many people, combat is the most traumatic event of their life.” Thus, many researchers have elected to use military personnel deployed to other places as controls when studying military exposures and health effects.
For the study of long-term health outcomes associated with exposure to burn pit emissions, the most appropriate comparison population would be deployed military personnel or veterans who have not been exposed to burn pit emissions either because they were stationed at military bases without burn pits or they were stationed at JBB before or after the burn pits were in operation. Recruiting a control group from this population would reduce the potential for a healthy-warrior effect, as both the exposed and comparison groups of deployed personnel would be similar in terms of their baseline health status.
As an alternative to using an unexposed control group, the committee also suggests a study based on comparisons between subgroups of deployed individuals with different degrees of exposure to burn pit emissions. If exposure can be estimated quantitatively, the wider the range of exposure, the greater the power of the study to detect an association, as described further below.
The committee believes the successful identification of exposed personnel and unexposed personnel deployed to sites without burn pits to be feasible. The DoD has already identified a cohort as evidenced in the report of epidemiologic studies (DoD 2010).
A previous IOM report stated “sufficient samples sizes for each cohort in the study are crucial to ensure adequate statistical power to find differences as well as to reliably identify the lack of differences between groups” (IOM 2000a). Sample-size calculations can be based on “expected magnitude of the difference between the exposed and unexposed groups, the relative sizes of the groups to be compared, and specified levels for type I error (the