1

Introduction

Military personnel may be exposed to many environmental and occupational hazards during deployment to a combat zone, including emissions from open burn pits, exhaust from jet and diesel engines, airborne dust and sand, solvents, and radiation. Exposure to some of these hazards is associated with mental, behavioral, and neurologic health outcomes. For example, evidence from civilian populations has shown that air pollution, which is similar in composition to some airborne military hazards, is associated with a range of mental health and neurodegenerative outcomes (Abolhasani et al., 2023; Borroni et al., 2022; Braithwaite et al., 2019; Cheng et al., 2022; Gong et al., 2023). Similarly, diesel exhaust has been shown to have neurodegenerative effects (Dickerson et al., 2018a,b; Goutman et al., 2022, 2023).

Military personnel returning from deployments have reported health problems and have also expressed concerns that these may be related to environmental and occupational exposures experienced during deployment. Studies have shown that some of these health problems are linked to deployment exposures (NASEM, 2016, 2020). Most research has focused on physical health effects, and less is known about the relationship between military-related environmental and occupational exposures during deployment and mental, behavioral, and neurologic outcomes. This report aims to provide evidence of whether selected environmental and occupational exposures during military service are possibly related to mental, behavioral, and neurologic health outcomes and chronic multisymptom illness (CMI).

STATEMENT OF TASK

In accordance with Section 507 of PL 117-168, Sergeant First Class Heath Robinson Honoring our Promise to Address Comprehensive Toxics Act of 2022 (PACT Act; see Appendix A), the Department of Veterans Affairs (VA) contracted with the National Academies of Sciences, Engineering, and Medicine to convene an ad hoc committee to assess possible relationships between environmental and occupational exposures during military service and mental health conditions and other health outcomes. VA further requested that the committee conduct an independent scientific assessment using information from Veterans Health Administration (VHA) health care records and other administrative records. At VA’s request, the report includes findings and conclusions but no recommendations. Box 1-1 provides the complete Statement of Task.

Interpreting the Statement of Task

At the committee’s first meeting, Dr. Patricia R. Hastings, Chief Consultant, Health Outcomes Military Exposures, presented VA’s charge to the committee (Hastings, 2024). She clarified that, consistent with the focus of the PACT Act, the committee should focus its assessment on veterans who deployed to the Southwest Asia Theater of Operations or Afghanistan. In this report, the committee follows VA and prior National Academies committees and defines the Southwest Asia Theater as Iraq, Kuwait, Saudi Arabia, the neutral zone between Iraq and Saudi Arabia, Bahrain, Qatar, the United Arab Emirates, Oman, the Gulf of Aden, the Gulf of Oman, the Persian Gulf, the Arabian Sea, the Red Sea, and the airspace above these locations (NASEM, 2020; VA, 2025). Approximately 3.7 million U.S. service members deployed to this region in the 1990–1991 Gulf War, a period of post-war stabilization between 1992 and 2001, and military campaigns after the September 11, 2001, terrorist attacks (NASEM, 2020). However, due to limited records from earlier conflicts available for analysis, Dr. Hastings added that the committee should focus on those who served in the post-9/11 conflicts.¹ Based on this population, the committee interpreted “toxic exposures” to refer to military-related environmental and occupational exposures that occurred during deployment to the Southwest Asia Theater of Operations or Afghanistan after September 11, 2001. In the general population, environmental exposures are typically low-level,

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¹ These include Operation Enduring Freedom (October 7, 2001–December 28, 2014), Operation Iraqi Freedom (March 19, 2003–August 31, 2010); Operation New Dawn (September 1, 2010–December 15, 2011), Combined Joint Task Force–Operation Inherent Resolve (October 17, 2014–present [as of August 2025]), and Operation Freedom’s Sentinel (January 1, 2015–August 31, 2021).

long-term hazardous exposures, and occupational exposures are hazardous exposures experienced in work settings or during work-related activities; military deployment-related exposures combine environmental and occupational exposures, because service members live and work in the same or nearby locations. Specific exposures of interest are defined in Chapter 3.

The conditions of interest specified in the committee’s task include traumatic brain injury (TBI). Because TBI is defined as “a traumatically induced structural injury and/or physiological disruption of brain function as a result of an external force” (VA and DoD, 2021, p. 6) and thus by definition is not caused by toxic exposure, Dr. Hastings clarified that the committee could include TBI in its analyses not as a primary outcome but as a secondary outcome or other type of variable. Although posttraumatic stress disorder (PTSD) is also not primarily caused by toxic exposures, but instead by exposure to death, serious injury, or sexual violence (NIMH, 2024), the committee retained it as a primary outcome, because individuals could have a predisposition to it. Together with exposure to toxicants during deployment, this underlying susceptibility may lead to PTSD onset following exposure to death, serious injury, or sexual violence. Such predisposition may also be relevant for the other outcomes of interest. Relatedly, for some of the conditions specified in the committee’s task, environmental and occupational exposures may not only cause onset of new disease but also exacerbate symptoms of preexisting conditions. Dr. Hastings clarified that the committee should focus only on new diagnoses in its assessment and not on symptom severity. She added that although the committee must include the conditions specified in its task, it could also consider additional outcomes; it added mental, behavioral, and neurologic health outcomes based on committee expertise and post-9/11 veteran interest in their etiology (see Chapter 4 for a full list of outcomes).

COMMITTEE PROCESS

The committee comprises expertise in biostatistics, environmental health, epidemiology, military and veterans’ health, neurology, psychiatry and psychology, and toxicology. Appendix C contains biographical sketches of the committee members. The committee held three information-gathering sessions. As described, the first focused on gathering input on its Statement of Task. The second was held to learn about potential VA and Department of Defense (DoD) data sources with information on military-related exposures and considerations for assessing selected health outcomes. At the third, the committee heard from veterans, advocates, and mental health providers to learn about veterans’ mental health problems, CMI, and concerns about military exposures. These sessions were immensely helpful in informing the committee’s deliberations around identifying and defining

relevant exposures and outcomes and appropriate sources of data for its analyses. See Appendix B for the session agendas.

Original statistical analyses using health care records and other administrative data provided by VA and DoD were conducted by a subcontractor (Westat) at the committee’s direction. In addition, the committee conducted a structured review of the epidemiologic literature on the associations between each exposure and outcome of interest to contextualize the results of its original analyses. It investigated possible relationships between 135 pairs of exposures and outcomes. Considerations for evaluating the evidence, including limitations of the data, and methods of analysis are presented in Chapter 4, and results of the committee’s data analysis and structured literature search are in Chapters 6–8.

COMMITTEE APPROACH TO EVALUATING THE EVIDENCE

The committee’s charge was to evaluate the evidence of possible relationships between military-related environmental and occupational exposures and mental, behavioral, and neurologic outcomes among veterans. Evidence came from two sources: (1) the results of the committee’s original statistical analyses using VA and DoD data; and (2) literature (published through 2024) from epidemiologic studies and, for certain exposure–outcome associations where the committee identified no epidemiologic studies, mechanistic studies providing a biologic and physiologic rationale. Before seeing the results of any analysis, the committee developed a framework for evidence synthesis, which it used to incorporate these two sources of information and develop conclusions about possible relationships between the exposures and outcomes.

The literature on evidence synthesis, systematic review, and meta-analysis of existing studies is vast, and no one definitive guide to evidence synthesis exists. Various paradigms offer approaches to combine the results of a current, original analysis with summaries of results and methods of previous studies. Several approaches are available to guide how much weight to place on previous studies that are not being used in a quantitative evidence synthesis approach. Ultimately, the committee decided to first consider results from its analysis and then use supporting evidence of findings from its structured literature review.

The reasons for pursuing this route were threefold. First, the committee’s charge was to conduct an independent scientific analysis using health records and other administrative data from VA and DoD. However, any conclusions drawn from a singular analysis may be limited by the availability and quality of the underlying data; as described in depth in Chapter 4, the committee was concerned about the quality of the available data and potential sources of bias from both systematic and random

error that could cause errors when interpreting its results. In particular, the committee recognizes limitations of its analyses related to the potential mismeasurement of exposures and outcomes and to gaps in the data on potential confounders; these limitations could have influenced the committee’s ability to observe the true relationships among the exposures and outcomes of interest. Limitations related to exposure measurement include only having data on exposures recorded by DoD, which are incomplete, periodic and inconsistent, and not documented systematically. In addition, the exposure data are recorded at the area rather than person level, which permit assessments of population risk rather than an individual’s risk of developing a condition. Further, assigning an individual’s exposure status based on area-level data may misclassify that person as exposed or not. These data also did not include reliable information on concentration, frequency, or duration of exposure, and the committee could not analyze concentration-response relationships. Outcome measurement was limited by only having information on conditions diagnosed at VA during 2017–2023, which does not capture outcomes diagnosed outside of VA or in VA before 2017. Additionally, as is common in electronic health record data, especially for psychiatric disorders, health conditions may have been overdiagnosed, underdiagnosed, misdiagnosed, or coded differently. Furthermore, data on potential confounding variables such as health conditions and traumas that occurred before military exposures were not available. These limitations of the data can both magnify or attenuate the observed possible relationship between exposures and outcomes, and thus lead the committee to mistakenly conclude that the relationship between a given exposure and outcome is either larger or smaller than the true relationship. Lastly, because the data were limited to DoD-documented exposures and VA-diagnosed outcomes, the committee’s sample only generalizes to the population of veterans who have a DoD-recorded exposure, have separated from the military, and who received VHA health care between 2017 and 2023. It does not generalize to all post-9/11 service members or veterans, including those who are still in the military.

The second reason for its approach is that for any given exposure–outcome association analysis, the committee did not examine just one single unique parameter. For example, for an association analysis, the committee reviewed multiple statistical measures, including the odds ratio estimates, uncertainty intervals, and stratification, and assessed the sensitivity of the inference to the choice of measured and unmeasured confounders. Third, the quality and strength of the epidemiologic literature across exposure–outcome pairs varies substantially, and without assessing the quality of each individual included study, pooling results to describe the general magnitude and direction of association may introduce bias and make the results somewhat subjective.

The committee therefore chose to present its original analyses and the literature summary separately and integrated them into a single conclusion using three hierarchical tiers of confidence of associations. First, it assessed the results of its own analyses using multiple statistical measures and considering the potential influence of systematic and random error to develop findings. Second, it assessed supporting evidence from the epidemiologic literature to determine whether its data analysis was aligned with results of similar exposure–outcome analyses in other populations. In the absence of epidemiologic literature, the committee considered mechanistic studies to inform a basis for a potential relationship. Finally, to develop its conclusions, it synthesized evidence from its analyses and the epidemiologic literature based on the strength and consistency of evidence across evidence types. This approach is described in more detail in Chapter 4.

ORGANIZATION OF THIS REPORT

Chapter 2 presents the committee’s conceptual framework and an overview of the constellation of factors that might influence the development of mental, behavioral, and neurologic health outcomes among veterans. Chapter 3 defines and describes the military-related exposures and how they may be related to select biological mechanisms. Chapter 4 presents the committee’s methods for its original data analyses and literature review and its process for synthesizing evidence from these sources to develop its conclusions. Chapter 5 describes the study sample. Chapters 6, 7, and 8 define and describe the outcomes; provide an overview of the burdens of these diseases and conditions among veterans; and present results for mental, behavioral, and neurologic health outcomes and CMI, respectively, from the committee’s analyses and epidemiological literature. Chapter 9 offers concluding remarks.

REFERENCES

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Dickerson, A. S., J. Hansen, O. Gredal, and M. G. Weisskopf. 2018a. Amyotrophic lateral sclerosis and exposure to diesel exhaust in a Danish cohort. American Journal of Epidemiology 187(8):1613–1622.

Dickerson, A. S., J. Hansen, M. A. Kioumourtzoglou, A. J. Specht, O. Gredal, and M. G. Weisskopf. 2018b. Study of occupation and amyotrophic lateral sclerosis in a Danish cohort. Occupational and Environmental Medicine 75(9):630–638.

Gong, Y., X. Zhang, X. Zhao, H. Chang, J. Zhang, Z. Gao, Y. Mi, Y. Chen, H. Zhang, C. Huang, and Z. Yu. 2023. Global ambient particulate matter pollution and neurodegenerative disorders: A systematic review of literature and meta-analysis. Environmental Science and Pollution Research 30(14):39418–39430.

Goutman, S. A., J. Boss, C. Godwin, B. Mukherjee, E. L. Feldman, and S. A. Batterman. 2022. Associations of self-reported occupational exposures and settings to ALS: A case-control study. International Archives of Occupational and Environmental Health 95(7):1567–1586.

Goutman, S. A., J. Boss, C. Godwin, B. Mukherjee, E. L. Feldman, and S. A. Batterman. 2023. Occupational history associates with ALS survival and onset segment. Amyotrophic Lateral Sclerosis and Frontotemporal Degeneration 24:219–229.

Hastings, P. 2024. Charge to Committee on the Mental Health Effects of Toxic Exposures Among Veterans. Presentation to the NASEM Committee on the Mental Health Effects of Toxic Exposures Among Veterans, Meeting 1 (February 22, 2024). https://www.nationalacademies.org/event/41865_02-2024_mental-health-effects-of-toxic-exposures-among-veterans-meeting-1 (accessed June 24, 2025).

NASEM (National Academies of Sciences, Engineering, and Medicine). 2016. Gulf War and health: Volume 10: Update of health effects of serving in the Gulf War, 2016. Washington, DC: The National Academies Press.

NASEM. 2020. Respiratory health effects of airborne hazards exposures in the Southwest Asia theater of military operations. Washington, DC: The National Academies Press.

NIMH (National Institute of Mental Health). 2024. Post-Traumatic Stress Disorder. https://www.nimh.nih.gov/health/topics/post-traumatic-stress-disorder-ptsd (accessed July 22, 2024).

VA (Department of Veterans Affairs). 2025. Gulf War Service. https://www.publichealth.va.gov/exposures/gulfwar/military-service.asp (accessed May 23, 2025).

VA and DoD (Department of Defense). 2021. VA/DoD clinical practice guideline for the management and rehabilitation of post-acute mild traumatic brain injury. Version 3.0—June 2021. Washington, DC: Department of Veterans Affairs and Department of Defense.