The charge of this committee is to assist the Department of Veterans Affairs (VA) with design of an epidemiologic study of potential long-term health effects of exposure to burn pit emissions, with specific reference to Joint Base Balad (JBB). The major challenges to the design of such a study are in the areas of exposure assessment and outcome ascertainment. This chapter describes feasibility and design issues associated with such a study. Available datasets on the potential long-term health effects of JBB burn pit emissions are discussed, as are limitations to these data.
Although the chapter focuses on an epidemiologic study of persistent health outcomes in military personnel and veterans associated with exposure to JBB burn pit emissions, the closure of the burn pit in 2009 precludes the collection of further data on concentrations of air pollutants at the time of the operation of the burn pit. Therefore, the committee also considers the possibility of an epidemiologic study of military personnel and veterans exposed to burn pit emissions at any U.S. military base with operating burn pits, not just JBB.
ELEMENTS OF AN EPIDEMIOLOGIC STUDY AND FEASIBILITY ISSUES
The elements of a well-designed epidemiologic study of the potential health effects of an environmental exposure include identification of a relevant study population of adequate size, comprehensive assessment of exposure, careful evaluation of health outcomes, adequate follow-up time, reasonable methods for controlling confounding and minimizing bias, and appropriate statistical analyses (IOM 2000a).
Epidemiologic Study Design Elements
The major types of epidemiologic studies are cohort, case-control, cross-sectional studies (study designs are discussed in detail in IOM 2000a). Most useful to investigating health effects from exposure to burn pit emissions would be a cohort study, such as the epidemiologic studies conducted by the Department of Defense (DoD) at JBB (AFHSC et al. 2010), in which the occurrences of health outcomes among all exposed personnel, ideally with quantitative individual measures of exposure, are compared to control (or unexposed) personnel with similar characteristics. Since the investigation may involve rare health outcomes such as brain cancer, the exposure history of personnel with a disease can be compared to that of non-diseased personnel in a case-control study. Cross-sectional (a snap shot of disease prevalence) and ecologic investigations (analysis of disease risk and exposure
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
error of rejecting the null hypothesis when it is true) and type II error (the error of failing to reject the null hypothesis when the alternative hypothesis is true)” (IOM 2000a). Several of the health outcomes potentially related to exposure to burn pit emissions identified in Chapter 6 (asthma, chronic obstructive pulmonary disease [COPD], and cardiovascular disease) are sufficiently common that if large numbers of military personnel or veterans deployed to JBB can be recruited, there should be sufficient power to study the effect of such exposure. Other outcomes of interest, such as various cancers or neurological diseases, may be too uncommon for there to be adequate power to assess whether they are associated with burn pit exposures.
The committee recommends that a pilot feasibility study be conducted to assess whether there will be sufficient power to study specific health outcomes given the currently unknown number of military personnel who might have been exposed to burn pit emissions.
Exposure assessment characterizes the frequency, magnitude, and duration of exposure to an agent of concern in a population. Accurately characterizing exposure is an essential step in conducting a well-designed environmental epidemiologic study. There are a variety of methods for collecting exposure information, but the most desirable is to measure exposures quantitatively at the individual level. Individual exposure measurements can be obtained through personal monitoring data or biomonitoring. However, if individual monitoring data are not available, and they rarely are, individual exposure data may also be estimated from modeling of exposures, self-reported surveys, interviews, job exposure matrices, and environmental monitoring. At JBB, the only environmental monitoring data currently available for the period when the burn pit operated are limited numbers of measured 24-hour average air concentrations (see discussion of environmental monitoring data for JBB in Chapter 4).
Long-term average concentrations are likely to be predicted by dispersion modeling, so that such modeling might be useful in estimating gradients across JBB in long-term average exposures. If further information on the time course of quantities of material burned becomes available (that is, if records are available, or if it can be confirmed that the total amount burned was approximately proportional to the numbers of persons on the base), such information could be incorporated in the dispersion modeling, although there is no guarantee of a linear relationship between emissions and quantities burned. Such estimates of a concentration gradient might be combined with location information for persons on the base (either self-reported or based on job and housing location) to estimate differential exposures to individuals.
Biomonitoring assesses an individual’s exposure to environmental agents by measuring the concentrations of the agents in biological samples, usually blood or urine but possibly adipose tissue, hair, or nails. The biomarker can be the external substance itself (for example, lead), or a metabolite of the external substance processed by the body (for example, cotinine, a metabolite of nicotine) and it indicates the absorbed dose or allows an estimate of target-tissue dose for the time of exposure. A chemical with a short half-life in the body might be detected for only a short time after exposure and be indicative of only recent exposure; whereas, chemicals with a long half-life that are not readily metabolized or excreted tend to stay in the body and may be indicative of past and cumulative exposure. For example, serum level of dioxin, which has a long half-life in the body, has been used as a biomarker of exposure to Agent Orange for some Vietnam War veterans (Henriksen et al. 1997). As described in Chapter 4, dioxins have been measured in the air at sampling sites near the JBB burn pit; these environmental measurements might be used as a marker of exposure to burn pit emissions because there is a concentration gradient with distance from the burn pit, taking wind direction into account.
Biomonitoring may be conducted using blood specimens that are available for military personnel through the DoD Serum Repository (see below). More research is needed to identify useful, meaningful, reliable, and implementable biomarkers and methods to measure exposure to dioxins and other chemicals in burn pit emissions. The value of serum dioxin as a biomarker of exposure for residents living near municipal incinerators has been studied in Spain and Taiwan, but serum levels were not well correlated with either measured atmospheric concentrations of dioxins or distance from the incinerators (Gonzalez et al. 2000; Huang et al. 2007). An initial attempt to use serum dioxin measurements in JBB personnel did not provide useful information (Taylor et al. 2008), and the predicted increments in serum concentrations based on an estimate of air exposures are low compared with background
serum concentrations in the general U.S. population. However, the serum dioxin measurements were severely compromised by high detection limits. The committee suggests that a paper exercise be conducted to estimate whether the expected small increment in serum concentrations, with the expected congener signature (based on the air measurements and the physiologically based pharmacokinetic model for dioxin), would be detectable if all 2,3,7,8 congeners could be accurately measured. If so, it would also be useful to determine if special efforts would now allow adequate detection of all the congeners in the small serum specimens available.
Personal monitors make it possible to directly measure exposures for a study population at the interface between the exposure medium and the human receptor (e.g., the breathing zone). In conjunction with records of the individuals’ activities, locations where the exposure to the highest concentrations might occur as well as the nature of emission sources can often be inferred. Personal monitoring methods are subject to various constraints, principally the availability of sensitive, cost-effective, easy to operate, sample collection equipment and analytic methods that provide sufficient time resolution and are free from interferences. In the case of JBB burn pit exposures, personal monitoring can no longer be employed because the burn pit is no longer operational. However, burn pits are still in operation at other military bases and personal monitoring at those bases, particularly for potentially highly exposed populations such as the pit operators, could be conducted.
Environmental monitoring is an indirect method of exposure assessment. Estimates of exposure may be made by combining measurements of pollutant concentrations at fixed sites with information on rates of contact with the medium of interest (e.g., air, water, soil) and recorded time-activity information. Modeling of air concentrations of particular chemicals at specified sites combined with workers’ job histories can be used to estimate the cumulative dose for each worker. At JBB, air pollutant concentrations at the three sampling locations, or based on dispersion modeling as described above, could be combined with information from personnel records on job type (activity), job location (that is, distance from the burn pit), and time stationed at JBB (duration of exposure) to assign burn pit exposures to individuals for an epidemiologic study.
Further environmental monitoring might be possible to evaluate gradients of exposure across the base; for example, it may be possible to evaluate the gradient in long-term average exposure to the burn pit by measurement of soil concentrations of PCDDs/Fs combined with the air dispersion modeling described above. The rate at which PCDDs/Fs deposit to soil depends on air concentration at ground level and the size of ambient particles on which the PCDDs/Fs condense (which particles are a complex mixture of the particles emitted by the burn pit and ambient particles from other sources). Once mixed in soil, PCDDs/Fs tend to be stable, but the mixing rate into the subsurface is location dependent. A pilot study to measure dioxin in soil across the base is likely the only approach that would definitively demonstrate whether gradients can be adequately established.
Compared with personal monitoring, use of environmental concentrations loses information because personnel at a given site may experience varied levels of exposure based on their activities, personal characteristics, and day-to-day differences in ambient pollutant concentrations; use of environmental concentrations assumes personnel are exposed to the average exposure at each site. Any approach that blurs the distinction between individual exposures while maintaining the collection of individual health outcomes will reduce the estimated variation in exposure and lead to exposure misclassification of some individuals. That biases the study, increasing the chance that any association between exposure and health outcome will not be detected.
Other approaches, such as self-report questionnaires and review of military-activity records by themselves, that is, those not linked to environmental monitoring, are unlikely to yield as accurate an assessment of exposure because of recall bias and greater potential for exposure misclassification. The usefulness of such historical information for exposure assessment might be enhanced with linkage to maps of JBB that display the location of the burn pit and the three sampling locations (mortar pit, guardhouse/transportation field, and CASF/H-6 housing). Unfortunately, the environmental monitoring conducted at JBB was done on an insufficient number of days (that is, sampling was only done on 53 days in 2007 and 2009 combined) to provide reliable estimates of long-term average exposures to burn pit emissions.
Given the limitations described, in the opinion of the committee, the sampling data described in Chapter 4 are sufficient only to allow ordinal categorization of exposure (that is, low, moderate, high) in combination with questionnaires and time-activity information. Improved quantitative estimates might be obtained if air dispersion and deposition modeling are consistent with gradients in soil dioxin measurements. Considering the lack of
currently available exposure information, additional environmental monitoring at other sites with operating burn pits will provide greater context for future studies as would biomarkers if they can be reliably correlated with environmental exposure. Feasibility will be greatly affected by the availability, reliability, and utility of the data sets and other information resources.
Evaluation of Health Outcomes
In addition to accurate and comprehensive exposure information, accurate ascertainment of the health outcome(s) of interest is necessary for the conduct of an epidemiologic study of the potential health effects of an environmental agent. A variety of methods for ascertaining health outcomes is available, including review of death certificates, medical records, and data from clinical examinations; linkage to disease registries; and use of self-reported outcomes from surveys or interviews.
Medical records can be a useful source of outcome data because health information in these records is usually recorded by trained health-care providers, although recording errors or misdiagnoses may occur. Death certificates are comprehensive in coverage but do not capture nonfatal adverse health outcomes, contain limited data, and coding errors are common. Like self-reported exposure information, self-reported outcome information may be biased because subjects do not always recall information accurately. It is therefore important to verify self-reported information through physician diagnoses, death certificates, or disease registries when feasible.
Complete and accurate assessments of health outcomes for exposed and control groups are important to evaluate whether an increase in the number of cases of a particular disease is related to exposure to burn pit emissions. That information can then be compared with the background rate of the disease in the control population to determine whether there is an “excess” of cases in the exposed population. Outcome assessments should be designed to minimize bias in ascertainment and have an adequate follow-up period to allow for the observation of outcomes with long latency periods, such as cancer.
Numerous studies have demonstrated that military personnel and their health status can be successfully identified for prospective and retrospective epidemiologic studies, for example, the Millennium Cohort Study (MCS) (see discussion of this study later in the chapter) and the recent epidemiologic studies conducted by the DoD (AFHSC et al. 2010). Given the many outcome assessment resources available to the DoD (described below), the committee believes the long-term follow-up of personnel exposed to burn pits to be a feasible task.
An observed association between burn pit exposure and a health outcome could be confounded by factors that are related to both the likelihood of exposure to burn pit emissions and the outcome. For example, military personnel who serve in roles that pose an increased risk of burn pit exposure may be more likely to be smokers, and smoking is associated with increased risk of respiratory disease, cardiovascular disease, and some cancers. The influence of confounders on the relationship between burn pit exposure and health outcomes can be reduced by careful study design and data-analysis schemes. Multivariable data-analysis techniques, such as stratified analysis and regression modeling, can effectively control for confounding by adjusting for multiple factors simultaneously (for example, age, smoking, and sex). However, the effectiveness of such methods depends in part on ascertaining potential confounding factors with accuracy and precision. Furthermore, controlling for confounding factors is of greater importance in situations where the magnitude of the association, in this case burn pit emissions and health outcomes, is modest relative to the relationship between the confounder and either burn pit exposure or the health outcome. For example, smoking is strongly associated with respiratory disease so it may be more difficult to attribute an increase in respiratory disease among exposed military personnel who smoke to burn pit emissions compared with nonsmokers.
Effect modification should be minimized in the design of an epidemiologic study. For example, sex would be an effect modifier if there were a substantial difference in the magnitude of the association between burn pit exposure and a health outcome for men and women. When such interactions are detected, effects must be estimated and reported separately by subgroup (that is, stratified) rather than adjusted for in the whole population,
as is appropriate for a confounder; such stratified analyses would require an even larger total study size to have adequate power to detect effects. Obtaining large numbers of study subjects poses a challenge for any epidemiologic study even in the absence of interactions.
While controlling for the influence of all factors is not practical, the DoD must attempt to minimize bias by controlling for the most influential confounding exposures. The committee suggests that control for confounding or effect modification should be considered and integrated into the study design. Of particular interest are behavioral habits (smoking, activity level), personal characteristics (age, race, sex), and other environmental or occupational exposures (for example, occupations with exposure to toxicants such as jet fuel). The feasibility of identifying and controlling for specific confounders should be addressed in a pilot study; the resulting data can be used to allow modification of the study design or adjustment of the analyses.
AVAILABLE DATASETS AND RESOURCES
Available datasets that may provide relevant information on either exposures to burn pit emissions or long-term health outcomes associated with such exposures are discussed below. Ongoing and planned studies of active-duty military personnel or veterans that can be used to assess long-term health outcomes associated with burn pit emissions are also described. The use of existing data or assessment efforts for future studies is helpful in reducing costs and enhancing study feasibility.
Exposure Assessment Information Sources
As described in Chapter 4, environmental sampling in the vicinity of the JBB burn pit was conducted from 2006 through 2009 by the U.S. Army Center for Health Promotion and Preventive Medicine (CHPPM, now the U.S. Army Public Health Command) (Taylor et al. 2008; CHPPM and AFIOH 2009; USAPHC 2010). No individual exposure data were collected at JBB. The burn pit was closed in 2009, and thus there are no further opportunities to collect environmental or personal monitoring data to assess exposure to burn pit emissions or other air pollution at JBB, except perhaps to measure concentrations of dioxin in soil and correlate those with air dispersion and deposition modeling.
However, other sources of exposure data do exist. Launched in 2001, the MCS is a coordinated, systematic 21-year effort to study the potential long-term health effects associated with deployment-related exposures (http://www.millenniumcohort.org/index.php, accessed November 11, 2010). The cohort consists of 152,000 consenting military personnel drawn from all branches of military service, including the Coast Guard (Ryan 2007), and representing approximately 11.3% of the 2.2 million men and women in service as of October 1, 2000. Recently, the MCS has focused on burn pit emissions at JBB and has been assigning exposures to military personnel stationed there by creating geographic buffer areas of varying radii (2, 3, 5, or 10 miles) around the burn pit. Those who resided within a specific buffer zone are considered to be exposed to burn pit emissions compared with those not residing within the buffer (DoD 2010). Duration of time spent at the JBB within the buffer zone can also be used to assess exposure for a study of long-term health effects. Estimates of exposure to JBB burn pit emissions could be enhanced by the inclusion of data on wind speed and direction, which are available for JBB. Land-use information could also be incorporated into the exposure assessment to provide a more complete characterization of possible sources of particulate matter present on the base.
The MCS also collects self-reported exposure data in its surveys of study participants. Beginning in 2010, the MCS survey has included a question on “exposure to burning trash.” Certain potentially highly exposed subgroups, such as personnel who regularly visited or who operated the burn pit, could be identified by further queries on exposure. Two DoD questionnaires, the Post-Deployment Health Assessment (PDHA) and the Post-Deployment Health Reassessment (PDHRA), are administered to all deployed military personnel upon their return to the United States and again 3–6 months later (available at http://www.pdhealth.mil/dcs/post_deploy.asp). Those questionnaires include a number of items about health outcomes and exposure, including “Are you worried about your health because you were exposed to ‘smoke from burning trash or feces’?” The Armed Forces Health Surveillance Center (AFHSC) collects the PDHA and PDHRA data (see section on Evaluation of Health Outcomes).
Another factor that affects the exposure of military personnel at a base with a burn pit is the type of housing available. While the committee has not done a systematic review of the housing at bases in Iraq and Afghanistan, it is likely that much of the housing would allow penetration of fine particles into the living spaces. Thus, military personnel would be exposed during off-duty time spent in their quarters as well as during duty hours. Further information on housing available on bases and indoor air monitoring would also help characterize potential exposures to burn pit emissions.
The DoD Serum Repository collects and stores specimens from military personnel; it currently houses over 50 million specimens linked to individual demographic data. The mission of the repository is “to receive and store remaining serum specimens from HIV testing programs within the DoD, and to receive and store serum specimens related to operational deployments worldwide” (available at http://afhsc.army.mil/dodsr). The AFHSC states that the repository can be used in epidemiologic studies and has been used historically as a screening tool to identify widespread infection.
A potential approach to exposure assessment for a future study of persistent effects of burn pit emissions is to measure one or more environmental agents or their metabolites (biomarkers of exposure) in samples of serum. One could also measure biomarkers of effect such as C-reactive protein or fibrinogen, known biomarkers of cardiovascular disease risk, assuming the study design allows for the biomarker of effect to be correlated with exposure.
The MCS can already be linked to the DoD Serum Repository. The repository has approximately four samples stored per individual, although for many individuals pre- and post-deployment samples are not available. The MCS is exploring the feasibility of using the Serum Repository samples for biomonitoring of military personnel deployed at the JBB (Smith et al. 2009).
Evaluation of Health Outcomes
Medical Surveillance of Active-Duty Military Personnel
The AFHSC is the central epidemiologic resource for the DoD; its main functions are to “analyze, interpret, and disseminate information regarding the status, trends, and determinants of the health and fitness of U.S. military (and military-associated) populations, and to identify and evaluate obstacles to medical readiness” (http://afhsc.army.mil/viewDocument?file=AFHSC_Brochure/AFHSC_Brochure.pdf). The AFHSC maintains a number of surveillance databases that are used to track the health of military personnel, including the PDHA and PDHRA described earlier. One such database is the Defense Medical Surveillance System (DMSS) that contains up-to-date and historical data on diseases and medical events (for example, hospitalizations, reportable diseases, and acute respiratory diseases, among others), longitudinal data on personnel and deployments, and the Standard Inpatient Data and Standard Ambulatory Data Records (available at http://afhsc.army.mil/dmss). AFHSC uses the DMSS to provide a link between the DoD Serum Repository and other databases. The DMSS records could be used to study health effects in those exposed to JBB burn pit emissions, although given the closure of the burn pit in late 2009, such a study could only be retrospective.
The DoD maintains a number of registries, including the Automated Central Tumor Registry and the DoD Medical Mortality Registry. The former registry is the DoD’s central registry for cancer and may be linked to other databases so cancer data can be tracked. The DoD Medical Mortality Registry, administered by the Armed Forces Medical Examiner System, collects complete medical and related information on every military active-duty death for surveillance and prevention purposes.
In addition to civilian disease and death registries, several databases supported by the VA that follow veterans after they have left the military would be useful for an epidemiologic study and assist in long-term follow-up.
The VA Beneficiary Identification and Record Locator Subsystem (BIRLS) is an automated system for iden-
tifying veterans and their beneficiaries who have received compensation, pension, education, or other VA benefits. The BIRLS database contains the BIRLS Death File. Veteran disability benefit claims for health outcomes potentially related to burn pit exposure (for example, respiratory diseases) can be obtained for veterans who have been identified as exposed to burn pits.
The VA Patient Treatment File (PTF) contains information on inpatient records for each discharge from a VA hospital facility since 1970 (updated biweekly). It can be used as a sampling frame to identify potential subjects for case-control studies. The PTF can also be used to assess health care utilization or morbidity for selected cohorts of veterans. The VA Outpatient Clinic File contains records from visits to outpatient VA clinics.
The VA maintains several health registries. The Cancer Registry is compiled from cancer registries maintained at each VA facility and contains records on all veterans receiving a diagnosis of cancer within the VA medical system. The VA also maintains registries of veterans diagnosed within the VA system with other conditions, including amyotrophic lateral sclerosis and multiple sclerosis.
The VA also supports a program called War-Related Illness and Injury Study Centers (WRIISC). The goal of WRIISC research is to better understand and improve the health of combat veterans. This research investigates topics of concern for combat veterans and their families, health care providers, and policy makers. WRIISC research interests include: environmental exposures and post deployment health and the long-term health effects of war.
The MCS described earlier has already been used to investigate the health outcomes among OIF/OEF personnel at bases with burn pits, but the followup in these investigations was limited (Smith et al. 2009). Use of the MCS is limited by its participation rate; initially enrollment was only 35%, although the follow-up rate is approximately 70–80%. Thus, the retained participants may represent a biased sample of military personnel, potentially different from military personnel who chose not to participate. The MCS is planning to enroll an additional 50–60,000 participants, bringing the total study population to 210,000. These new enrollees will be asked to donate biological samples for biomarker assays (Smith et al. 2009).
Outcome assessment is based on self-reports, although the MCS can also access DoD inpatient, pharmaceutical, and vaccination data through the Defense Medical Surveillance System. Unfortunately, those data are only available for active-duty military personnel. At present, the MCS is not able to access health outcome data in VA databases.
The VA is conducting The National Health Study for a New Generation of U.S. Veterans, a 10-year longitudinal study of 60,000 OIF and OEF veterans. A pilot study with 3,000 participants was completed in spring 2009; the response rate among contacted eligible veterans was about 30%. The full-scale study began with a questionnaire in 2009. Exposure and outcome data are entirely self-reported and based on a series of question about types of health care, diagnoses, and a variety of exposures, including a question about exposure to burning trash or feces, oil fires, and smoke. The survey also collects information on branch of the military, component, and job title, and asks about where the respondents were stationed since 2001 and the total number of times deployed. The validity and reliability of the questionnaire is being studied.
PROPOSED APPROACHES TO THE STUDY OF HEALTH OUTCOMES FROM EXPOSURE TO BURN PIT EMISSIONS
The committee recommends a cohort study of the long-term health effects (evaluated prospectively) from retrospective estimates of exposure to burn pit emissions in military personnel deployed at the JBB. To determine the incidence of chronic diseases or cancers with long latency, individuals must be followed for many years. Ideally, the observation period for health effects begins retrospectively, at first deployment to JBB, and continues after active duty is completed. First, however, the committee strongly recommends that pilot studies be conducted to address issues of statistical power and develop design features for specific health outcomes. It is important to note that once a prospective cohort infrastructure has been established, multiple health outcomes can be studied in the cohort over time. Intermediate outcomes on the pathway to the development of chronic diseases can also
be studied in a serial manner. For example, serial spirometry can be used to detect excessive rates of decline in lung function before a diagnosis of COPD is made and serial measurement of intima-media thickness in carotid arteries can be used to detect early atherosclerosis.
Retrospective estimation of exposure will prove particularly difficult. To characterize exposures to the complex mixture of burn pit emissions while accounting for other environmental and occupational hazards including air pollutants from other sources, the committee recommends a tiered approach. Since direct quantitative exposure measurements are not possible, the committee sought to outline general study designs that could easily be informed using available data previously described. The three tiers of the recommended study are characterized by the decreasing specificity of exposure and would answer different research questions, as follows:
- Tier 1: Did proximity to burn pit operations at JBB increase the risk of adverse health outcomes?
Ordinal estimates of individual-level exposure to JBB burn pit emissions can be determined based on dates of deployment, duties on base, and location of housing relative to the burn pit, taking account of wind-dispersion models. The exposure effect can be assessed by comparing subgroups with more and less exposure among all individuals with potential exposure, that is, those stationed at the JBB during the period of full burn pit operation (2003–2007). The use of soil dioxin concentrations at various locations at JBB should be considered as a potential marker of exposure to burn pit emissions.
- Tier 2: Did installation of incinerators at JBB reduce incidence of disease or intermediate outcomes (for example, emphysema or rate of lung function decline)?
Assess exposure (yes/no) to JBB burn pit by date of initial deployment. This approach considers the installation of incinerators to replace the burn pit as an intervention. Comparisons can be made between post-deployment chronic health outcomes among those deployed to JBB before the burn pits were shut down and those deployed there afterwards. A caveat here is that the installation of incinerators at JBB occurred in phases. In July 2007, two incinerators were put into operation at JBB, and in April 2008 a third incinerator began operation. By October 2009, burn pit operations at JBB ceased when the fourth incinerator began operating, resulting in 100% solid waste disposal via incineration or off-site recycling. Thus, it may be necessary to study three different times defined by the beginning and end of the incinerator installation period: before July 1, 2007; July 1, 2007, to October 1, 2009; and after October 1, 2009.
- Tier 3: Did deployment at JBB during full burn pit operation increase the risk of adverse health outcomes compared with deployment elsewhere in Iraq or Afghanistan or with no deployment?
Assess exposure (yes/no) to the total JBB environment, recognizing that the burn pit emissions occurred in the presence of PM and other air pollutants from other sources. This broad definition of exposure can be assessed by comparing the health experience of military personnel deployed at the JBB during the period of burn pit operation to that of military personnel deployed to Iraq and Afghanistan at locations without a burn pit or that of military personnel not deployed to the Middle Eastern theatre during the same time. This approach was used by AFHSC and MCS to conduct short-term health studies described in Chapter 6 (AFHSC et al. 2010). Although there are limitations to this approach, it may be possible to find an appropriate unexposed comparison group—preferably another deployed population unexposed to burn pits but exposed to PM and other chemicals identified at JBB from other sources. The recommendation for a nondeployed comparison group is based on the committee’s judgment that pollution in the region from sources other than burn pits may pose greater health risks than burn pit emissions.
Elements of a three-tiered prospective cohort study of active-duty military personnel and veterans designed to assess potential chronic health effects related to burn pit emissions are presented in Table 8-1.
TABLE 8-1 Proposed Nested Prospective Cohort Studies of Long-Term Health Effects
|1||Did proximity to burn pit at JBB increase risk of adverse outcomes?||Deployed to JBB during burn pit operation (2003–2007)||Estimated individual level ordinal exposure to burn pit emissions based on JBB job, barracks location, and duration of JBB deployment. Exposure: low, medium, high|
|2||Did installation of incinerators at JBB reduce disease incidence or rate of change in intermediate outcomes?||Deployed to JBB before, during and after burn pit operation (2003–2009)||Deployed to JBB before, during, or after incinerator installation. Exposure: yes or no|
|3||Did deployment at JBB during full burn pit operation increase risk of adverse outcomes compared to deployment elsewhere in Iraq or Afghanistan or compared to nondeployed?||Military service during 2003–2007 (deployed and nondeployed)||Deployed to JBB with burn pit, deployed to Iraq or Afghanistan sites without burn pits, or nondeployed. Exposure: yes or no|
The committee acknowledges that greater specificity in study design is accompanied by greater limitations on data collection and decreased feasibility in conducting the study. Feasibility issues for an epidemiologic study to determine the long-term consequences of exposure to burn pits in Iraq and Afghanistan include the inability to obtain direct, individual exposure measurements for personnel at JBB as the burn pit has been closed since 2009. However, it might be feasible to obtain job title, duties, and base location in addition to the location of barracks to estimate individual exposure. From previous studies, the most simplistic determination of exposure to burn pits, that is deployment to a site operating a burn pit (yes or no), is feasible as described in Tier 3 (DoD 2010). Furthermore, as noted earlier, there are other military bases with operating burn pits, particularly in Afghanistan where personal monitoring could be conducted.
The main determinant of study feasibility is access to complete, accurate, reliable data pertaining to deployment, demographic and personal characteristics, and health outcomes from the DoD, the VA, and civilian sources. The committee recommends a pilot study be completed to examine feasibility including data availability, power, exposure assessment, confounding, and assessment of health outcomes.
CONCLUSIONS AND RECOMMENDATIONS
As outlined by the committee’s statement of task, several important aspects of epidemiologic study design and their feasibility have been discussed. Considering the feasibility issues, the committee has developed the following recommendations for a potential study of long-term health effects associated with exposure to burn pits:
- A cohort study of veterans and active duty military should be considered to assess potential long-term health effects related to burn pit emissions in the context of the other ambient exposures at the JBB. This type of study, while complex, is not unique in a military setting (for example, standard methods exist for the U.S. Air Force Ranch Hand Study that examined health effects of Agent Orange).
- An independent oversight committee composed of military and external experts in air pollution, analytical chemistry, exposure assessment, epidemiology, toxicology, biostatistics, and occupational and environmental medicine should be established to provide guidance and to review specific objectives, study designs, protocols, and results from the burn pit emissions research programs that are developed. Such a committee
could provide an essential peer-review function to lend greater scientific credibility to the investigations. An example is the advisory committee that was established to oversee the conduct of the Ranch Hand Study (IOM 2006).
- A pilot study should be conducted to ensure adequate statistical power, ability to adjust for potential confounders, to identify data availability and limitations, and develop testable research questions and specific objectives. The objectives should be used to motivate essential study design features. Examples of these features include: subject eligibility criteria, size and demographic characteristics of the cohort, length of follow-up required, health outcomes to be studied, critical time periods of exposure, and potential confounding and modifying factors that would need to be measured. Careful consideration should be given to defining sensitive and useful exposure measures.
- Assessment of health outcomes is best done collaboratively using the clinical informatics systems of the DoD and the VA, in addition to the non-military methods of follow-up (for example, National Death Index, state cancer registries) that can be used to identify the incidence and prevalence of health effects over time. Integration of current programs, such as the MCS, would increase feasibility and ease of study initiation. Multiple health assessments in the form of questionnaires and specific medical assessments could be administered periodically to better address intermediate and non-fatal health outcomes.
- An exposure assessment for better source attribution and identification of chemicals associated with waste burning and other pollution sources at JBB should be conducted prior to beginning a new epidemiologic study to help the VA determine those health outcomes most likely to be associated with burn pit exposures. The committee’s analysis of available data from the environmental monitoring conducted at JBB suggests that exposure to PM emitted from sources such as diesel and jet engines, upwind Iraqi urban areas, and soil, may be of greater concern than exposure to burn pit emissions.
- Exposure assessment should include detailed deployment information including distance and direction individuals lived and worked from the JBB burn pit, duration of deployment, and job duties. Multiple methods of estimating exposure have been discussed; however, the most applicable method should be defined by the study questions, data availability and limitations, and study design. Study of troops currently deployed at bases with operating burn pits, in addition to JBB, would allow for prospective exposure assessment of those troops and provide information useful to interpretation of results from JBB.
In conclusion, a study of health effects resulting from exposure to burn pits is feasible but its ability to produce useful and actionable results depends on a well thought-out design, thorough exposure assessment and careful follow-up. The IOM and NRC have recommended several methodologies for investigating and monitoring the health of military service members to the VA and the DoD over the years that, in addition to this report, can provide further guidance on study design and feasibility (IOM 1999, 2000b, 2008b; NRC 2000a,b).
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