3

Military-Related Environmental and Occupational Exposures

This chapter describes the military-related environmental and occupational exposures the committee included in its analysis; it also includes information on how these are related to mechanisms (oxidative stress, neuroinflammation, and neurotoxicity) identified by the committee that contribute to its outcomes of interest. Environmental exposures can be physical, chemical, or biological substances and are typically low-level, long-term hazardous exposures experienced by the general population in the home environment, in the work environment (incidental exposures and not job related), and through daily life activities. Possible exposure can occur through air, water, soil, food, and consumer and personal care products; hazards can be ingested, inhaled, or absorbed (NCI, n.d.). Occupational exposures are potentially hazardous exposures experienced in work settings or during work-related activities, such as agricultural work, construction, welding or working with metals, or textile processing (NIEHS, 2023). They occur during work hours, though some may be carried home without proper use of personal protective equipment and other precautions. Regulations on occupational exposures are set by Occupational Safety and Health Administration, and assessments are more common. Military exposures are special circumstances, as they combine environmental and occupational exposures because deployed service members are working and living in the same locations or in close proximity. Given the nature of deployment and war-time events, exposures tend to be more intense, with a higher frequency of contact and often for the duration of the deployment. Additionally, deployment-related exposures tend to be to multiple contaminants and not a single exposure at a time.

As discussed in Chapter 1, the committee was given latitude in defining “toxic exposures” and defined military exposures in the post-9/11 era in the Southwest Asia Theater of Operations or Afghanistan. Specific exposures were chosen based on the charge to committee from the Department of Veterans Affairs, committee expertise, and input from experts at the Individual Longitudinal Exposure Record who were familiar with the exposures captured by that dataset. The final exposures used for the analysis were determined by exposure data received by the committee and committee expertise and are described in this chapter. They include burn pits (capturing burn pits, burn barrels, and burning landfills), dust (including desert and road dust, particulate matter [PM], and specific pollutants such as volatile organic compounds [VOCs] and polycyclic aromatic hydrocarbons [PAHs]), diesel and jet exhaust, diesel and jet fuels, incinerator emissions, metals (including aluminum, hexavalent chromium, lead, manganese, and depleted uranium), mold, radiation, and solvents.

To select exposures of interest, the committee considered a wide array of exposures including broad sources such as exhaust and specific toxicants such as benzene identified in the literature as military-related (IOM, 2000, 2003, 2005). It then refined exposure categories based on the available data. Some exposures of initial interest due to their known cognitive effects and prevalence in the military (e.g., pesticides, blast exposures, ultrafine particles, per- and polyfluoroalkyl substances) were subsequently excluded due to data gaps (i.e., low prevalence or no data) or insufficiencies (e.g., substantial inconsistencies in the data). Other exposures initially categorized as separate exposure groups were subsequently consolidated into single exposure categories, because they were frequently documented together and isolating their unique effects would not be possible. For example, solvents, oils, and cleaners were grouped into a single solvent category. In addition, some categories were consolidated after preliminary analyses showed that there were no meaningful differences when assessed separately and together (e.g., jet fuel and diesel fuel were grouped as fuel). Chapter 4 describes the exposure data, the committee’s decisions around available datasets, and limitations of the exposure data in more detail.

BURN PITS

Burn pits are open-air waste-burning sites that were a primary solution for military waste management in post-9/11 conflicts in Iraq and Afghanistan. A 2009 law restricted their use, though they continued at some sites through 2019. Waste burned was inconsistent across sites, and may have included plastics, wood, metals, and other combustible and noncombustible materials. Petroleum, oil, lubricants, and other fuels were often used for initial combustion. Burn pit emissions (including the constituents

and their concentrations) depend on the specific waste but generally included high concentrations of PM, polychlorinated dibenzo-p-dioxins and dibenzo-p-furans, VOCs, and PAHs (NASEM, 2020). Burn pits and resulting inhalable emissions represent potential environmental and occupational exposures, and service members could have been exposed by living or working near them or on bases with them or through occupations involved in waste management, such as tending burn pits.

DUST AND PM

Dust can refer either to crustal dust (i.e., from Earth’s crust) or road dust, which includes resuspended particles from construction and tire and brake wear; dust mobilization therefore occurs from human activity and environmental and meteorological conditions (EPA, 2019). PM is a complex mixture of liquid droplets and solid particles comprising acids (e.g., nitrates and sulfates), organic chemicals, metals, soil or dust particles, and other components. Particles can be primarily emitted in the atmosphere as crustal dust or through processes including fossil fuel combustion, motor vehicle-related emissions (e.g., tailpipe emissions, break and tire wear), industrial processes, and wildfires (EPA, 2019). Particles can also be secondarily formed in the atmosphere given high temperatures and sunlight through photochemical reactions (e.g., secondary organic aerosols). PM is usually grouped into size-dependent categories: coarse (PM₁₀; aerodynamic diameter 10 μm) and fine (PM_2.5; aerodynamic diameter 2.5 μm) particles (NASEM, 2020).

Coarse particles (PM₁₀), are a potential environmental exposure that comprises a mixture of liquid and/or solid particles suspended in air, and may include chemicals, metals, dust, and soil, depending on the local environment and industries (California Air Resources Board, n.d.; EPA, 2019, 2022; NASEM, 2020). Service members may be exposed through deployment to sandy and dusty environments, such as in the Persian Gulf and Afghanistan, or local civilian or industrial pollution, such as from vehicle emissions, power plants, agriculture, and industrial operations (NASEM, 2020).

Fine particles (PM_2.5) can reach deeper into the respiratory system (NASEM, 2020; VA, 2025e). PM_2.5 is a subset of PM₁₀ and is therefore also a potential source of environmental exposure made up of a mixture of liquid and/or solid particles suspended in air, including various substances depending on the local environment (ARB, n.d.; EPA, 2019, 2022; NASEM, 2020). Service members can be exposed to PM_2.5 by inhaling combustion products and by-products from a range of military and civilian sources such as waste burning, vehicles, generators, and local industry (NASEM, 2020).

Road Dust

Road dust is a potential source of environmental exposure and refers to sand, dust, soil, and other particles on roads and airfields that are suspended in air through aircraft or motor vehicle disturbance of unpaved roads or the desert floor. In the military, service members may be exposed to road dust via inhalation and through occupations involved in aircraft or vehicle transport and maintenance.

Desert Dust

Desert dust refers to sand, dust, soil, and other particles suspended in ambient air or from dust storms and sandstorms, and anthropogenic emissions (e.g., from vehicles and industrial or agricultural operations). Desert dust is an inhaled environmental exposure, and sources of military exposure similarly include deployment to sandy and dusty environments or local civilian or industrial pollution.

FUELS

Fuels, or fuel oils, are substances designed to power engines, including those used to propel aircrafts and other vehicles.

Jet Fuels

Jet fuels, also referred to as jet propellants or aviation turbine fuels, are complex mixtures composed primarily of kerosene, aliphatic and aromatic hydrocarbons, and nonhydrocarbon performance additives. The military primarily uses JP-5 and JP-8 (ATSDR, 2017; NRC, 2003; VA, 2023). Other military-related uses include fueling nonaircraft engines, such as generators, tent heaters, and stoves; as a solvent for cleaning or degreasing; and igniting burn pits (NASEM, 2020; VA, 2023). Engines may aerosolize unburned jet fuels. This occurs especially when cold, because of the higher flash point and lower volatility of jet fuels compared to other fuels (NRC, 2003). Jet fuels are a potential occupational exposure, and routes include dermal contact, inhalation, and ingestion. Service members may be exposed via accidental spills or through occupations that transport, store, and maintain jet fuel; operate fuel generators; are in close proximity to burn pits; or are on military bases with aircraft or on military aircraft carriers (VA, 2023).

Diesel Fuels

Diesel fuel is a fuel oil refined from crude petroleum. It is used in a variety of ways, including in vehicles, lamps, heaters, furnaces, stoves, and generators and also as a solvent (ATSDR, 1995). It is commonly used in deployment because it is less flammable than other types of fuels (VA, 2025b). Like jet fuel, diesel fuel is an occupational exposure with dermal, ingestion, or inhalation routes. Sources of potential military exposure include accidental spills and occupations that transport or store diesel fuel or fuel vehicles and machinery (VA, 2025b).

EXHAUST

Exhaust is a mixture of gases and particulates produced during combustion, especially from engines. Engine emission composition varies among engine types, fuel formulations, and operating conditions (e.g., acceleration, deceleration, idle) and source (vehicle or nonroad engines). However, gases typically include PAHs and VOCs; PM may include carbon, metals, and other trace elements. Upon emission from an engine, exhaust can be diluted, transformed (chemically or physically), and dispersed in the atmosphere (EPA, 2002).

Jet Exhaust

Jet engine exhaust is a mixture of gases and particles produced during the combustion of jet fuel. It is an inhalable potential source of environmental or occupational exposure. Sources of military exposure include occupations that transport, store, and maintain jet fuel; operate fuel generators; tend to burn pits; or are on military bases with aircraft or on military aircraft carriers (NASEM, 2020).

Diesel Exhaust

Diesel engine exhaust is a mixture of gases and particles produced during the combustion of diesel fuel. Like jet engine exhaust, it can be inhaled and is an environmental or occupational exposure. Service members can be exposed from ambient air pollution or through occupations that use diesel engines (including on-road and nonroad engines), such as motor vehicle transport.

INCINERATOR EMISSIONS

Waste incineration is a method of waste management that involves combustion of solid waste in a furnace, which produces gases and ash residue for disposal. Cooled gases are typically treated to remove pollutants and dispersed through an induced-draft fan and stack (NRC, 2000). The specific combustion products emitted from incinerators and their concentrations depend on the waste burned but may include carbon monoxide, carbon dioxide, nitrogen oxides, sulfur oxides, metal oxides, metal particulates, and PAHs. Incinerator emissions can be inhaled and are environmental or occupational exposures. Military exposure sources include living or working on bases with an incinerator or occupations in waste management.

METALS

Metals are primarily naturally occurring substances in Earth’s crust and can be pure elements or alloys. Generally characterized by high electrical and thermal conductivity, they are commonly ductile and malleable with a lustrous appearance and varying densities. Metals can be environmental or occupational exposures; uses include construction, transportation, electronics, and domestic goods.

Aluminum Production/Smelting

Aluminum metal is a naturally occurring element with applications ranging from construction to food and pharmaceutical uses. The industrial aluminum production process, known as “aluminum smelting,” produces particulates and fumes that may include toxicants such as PAHs, sulfur dioxide, carbon monoxide, carbon dioxide, and trace metals (IARC, 2012; Wesdock and Arnold, 2014). This is a potential source of environmental exposure, and service members can be exposed to the resulting particulates and fumes from nearby industrial aluminum production facilities.

Lead

Lead is a naturally occurring metallic element that has been used in gasoline, batteries, solder and pipes, pottery glazes, caulking, paint, and other commercial products. It is also emitted in ambient air from leaded gasoline used in commercial aviation and other fuel combustion, industry, and wildfires. In the military, lead is used in explosives, artillery, and ammunition (ATSDR, 2020; VA, 2025d). An environmental or occupational exposure can occur through ingestion or inhalation, such as from indoor

firing ranges; water from lead pipes; deteriorating lead-based paint; or the ambient air, soil, and dust.

Hexavalent Chromium

Hexavalent chromium is a form of the naturally occurring element chromium. It is used to make alloys like stainless steel to harden and increase resistance to corrosion and in electroplating as a coating. It is also added to paints, primers, and other coating materials to reduce corrosion (ATSDR, 2012a; NIEHS, 2024; OSHA, n.d.). It has dermal, inhalation, or ingestion routes. Service members may be exposed through occupations that involve welding or cutting metals or handling paints, coatings, or pigments.

Manganese

Manganese is a naturally occurring metal that is an essential element in humans. It is involved in metabolism, cellular protection, and bone hardening (ATSDR, 2012b). While essential for humans, higher levels of exposure beyond those necessary for biological processes have been found to be associated with various neurologic, endocrine, and immunological disruptive effects (among others). Routes of exposure can include inhalation, oral, or dermal.

Welding

Welding is joining metals, typically through melting and fusing through cooling. This process creates smoke composed of metal fumes and gases, which can be inhaled (OSHA, 2013). The metal fumes may include metals such as cadmium, chromium, lead, and manganese, while the gases may include carbon monoxide, carbon dioxide, nitrogen dioxide, and ozone. Service members can be exposed through welding occupations such as those related to vehicle or aircraft maintenance or construction.

Depleted Uranium

Depleted uranium is a weakly radioactive by-product created when a radioactive isotope is removed from uranium, the naturally occurring element, during nuclear fuel production (Health.mil, 2024; VA, 2025a). It has high density, a high melting point, and easy ignition. The military primarily uses it in ammunition and tank armor (IOM, 2008; U.S. NRC, 2023). Routes of exposure include inhalation and ingestion. Service members may be occupationally exposed to depleted uranium from friendly fire incidents,

proximity to burning vehicles or munitions containing depleted uranium, or vehicle salvage operations (IOM, 2008).

MOLD

Mold is a range of microscopic fungi that can be found ubiquitously in the natural environment, including routinely in air and on surfaces indoors. Spores and fragments that settle and grow on indoor surfaces can be suspended into the air, because of cleaning, occupant movements, or other disturbances, and inhaled (IOM, 2004). Mold is a potential source of environmental exposure, and military sources are from living or working in buildings or tents with mold growth.

RADIATION

Radiation is the emission or transmission of energy in the form of waves or particles. It can be classified as ionizing or nonionizing based on its energy and ability to ionize atoms or molecules. Sources include natural background radiation (e.g., cosmic rays, radon decay) and anthropogenic activities (e.g., medical imaging, nuclear power generation) (ATSDR, 1999).

Ionizing Radiation

Ionizing radiation is energy in the form of electromagnetic waves of radioactive particles that can remove electrons (ionization) from molecules of materials such as air, water, protein, and living tissue. It occurs naturally at low levels from space, Earth, and some materials. It is used in a variety of applications, including X-rays, computed tomography scans, positron emission tomography scans, and nuclear medicine, and can also be a by-product of generating nuclear power (ATSDR, 1999). It is an occupational exposure with a direct route. Sources of military exposure include occupations using X-ray technologies, such as medical (e.g., dental or X-ray technicians) and nonmedical (e.g., operating vehicle screeners used to detect explosives and weapons) applications and those involved in handling nuclear weapons (including testing, maintenance, and cleanup). Service members may also be exposed during service on nuclear ships (including submarines) or in shipyards, or from depleted uranium (VA, 2015, 2024).

Nickel-63

Nickel-63 is a radioactive isotope of nickel used to detect explosives, hazardous chemicals, and vapors (Oak Ridge National Laboratory, 2011). In the military, some chemical agent detectors contain sealed nickel-63. Routes

of exposure are direct, inhalation, and ingestion, and service members may face occupational exposure through its use in chemical agent detection devices. These devices contain sealed nickel-63, so service members could be exposed if a detector was damaged or leaked (DHA, 2024).

SOLVENTS

Solvents are a large, diverse group of chemicals that can dissolve other chemicals and are occupational exposures. In military settings, they include acetone, benzene, ethanol, glycerol, isopropyl alcohol, methanol, propylene glycol, and tetrachloroethylene. They are frequently used in occupational settings for cleaning and degreasing (IOM, 2003). Other routine tasks also may expose service members to solvents, such as stripping or thinning paints (VA, 2025c). Routes of exposure include dermal contact, direct eye contact, ingestion, and inhalation.

BIOLOGIC PLAUSIBILITY AND MECHANISMS

Biological plausibility, a well-accepted concept used commonly in environmental health and public health, is an important consideration when determining the relationship between exposures and outcomes. It can be used, with other evidence, to assess confidence in the results of a study of the relationship between an exposure and outcome; it can be particularly helpful when information is missing, data on exposure or outcome are incomplete, or bias exists in the data in epidemiologic or environmental studies (Savitz, 2021; Whaley et al., 2022). The underlying biological mechanism varies in each biological plausibility scenario, with some overlap across the committee’s exposure–outcome pairs of interest. When considering outcomes related to the nervous system, three main mechanisms have been shown to play a role through direct and indirect effects. Direct effects generally occur when the neurotoxicant crosses the blood–brain barrier and causes direct cellular damage or neuroinflammation. Indirect impacts have been found to occur through systemic inflammation and/or oxidative stress.

Although many of the mental and behavioral health outcomes reported by service members have distinct features, they also share many features, a characteristic that may relate to their comorbidities. This includes associations with both neuroinflammation and oxidative stress (Apweiler et al., 2024). Neuroinflammation is an inflammatory response within the brain or spinal cord resulting from activation of microglial cells and the production of cytokines, chemokines, and reactive oxygen species (i.e., a neuroimmune response of the nervous system) (Kolliker-Frers et al., 2021). Oxidative stress reflects an inability of the brain antioxidant response system to block reactive oxygen species, leading to cellular damage (Salim, 2017). Both

mechanisms, for example, have been implicated in depressive disorders (Sipahi et al., 2023).

Many of the exposures related to military service have been associated with neuroinflammation and/or oxidative stress (Brooks et al., 2024). Air pollution and PM, which have been most extensively studied, are considered inflammatory stimuli. Studies in both humans and animal models have demonstrated neuroinflammatory effects of air pollution as mediated by microglial activation (Costa et al., 2020; Jayaraj et al., 2017). In addition, air pollution has been linked to oxidative stress in multiple organ sites (Lodovici and Bigagli, 2011) and the brain in numerous studies in animal models (Bernardi et al., 2021; Calabró et al., 2021; Cheng et al., 2016). Evidence exists for biologically plausible pathways through multiple mechanisms that underly the detrimental effects on the nervous system from long-term exposure to PM_2.5; a causal relationship is likely between long-term PM_2.5 exposure and nervous system effects, likely owing to mechanisms involving neuroinflammation and/or oxidative stress (EPA, 2019). Some studies have examined such mechanisms in response to diesel exhaust exposures, which are ultimately a component of air pollution, with similar findings (Cole et al., 2016; Ehsanifar et al., 2019, 2021). Some evidence also exists of the neurotoxicity from exposure to uncombusted petroleum fuels. Studies indicate that this may occur by all routes of exposure and that fuel oils may be neurotoxic, owing to the central nervous system depression they cause (ATSDR, 1995). Further indirect evidence exists of health effects, including neurotoxicity, of inhalation exposure to JP-5, JP-8, and Jet A fuels (ATSDR, 2017). Military burn pits represent an even more toxic mixture than air pollution in that they involve open-air burning of materials such as plastics, metals, chemicals, and medical waste (NASEM, 2020). Their emissions can include, as with air pollution, PM, VOCs, heavy metals, dioxins, and PAHs but in even higher concentrations.

PAHs are a typical organic contaminant of air pollution. PAHs have been associated with markers of oxidative stress in humans in occupational contexts and population studies (Hu et al., 2021; Ryu and Hong, 2024; Wu et al., 2022). Recent studies have reported that PAHs were associated with reduced cortical thickness in adults over 50 years of age without dementia as well as decrements in learning and memory (Cho et al., 2020, 2023).

Organic solvents represent another class of chemical exposures widely used by the military. While data on individual organic solvents and their neurological effects are limited, experimental studies in animals have reported increases in oxidative stress markers in the brain after both acute and subchronic toxicity of solvents such as toluene (Kodavanti et al., 2015). Exposures of mice to low doses of volatile organic mixtures were likewise found to increase levels of reactive oxygen species in the brain in addition to alterations in brain neurotransmitters (Wang et al., 2014). Numerous

studies over the years have reported organic brain damage in workers exposed to solvents, including chronic solvent-induced encephalopathy (van Valen et al., 2018; Visser et al., 2008).

Similarly, studies of depleted uranium in animal models have reported its pro-oxidant activity after drinking water exposures, including increases in lipid peroxidation and alterations in gene expression of antioxidant enzymes, and in metal transporters in the cerebral cortex (Briner and Murray, 2005; Lestaevel et al., 2009). For example, in studies with rats, fragments of depleted uranium have been found in the brain following surgical implantation of depleted uranium pellets in the thigh. Researchers have also found distribution of depleted uranium from the respiratory tract of rats to secondary organs, including the brain, following exposure to repeated inhalation; this includes intranasal transport from the nasal mucosa to olfactory axons, which bypasses the blood–brain barrier. Affected brain areas include the olfactory bulb, hippocampus, and frontal cortex (Ibanez et al., 2014; Monleau et al., 2005; Pellmar et al., 1999; Petitot et al., 2013).

Hexavalent chromium has also been identified as a neurotoxicant that, in addition to its association with carcinogenicity in humans, has been shown to produce oxidative damage and redox stress in animal models (Wise et al., 2022), with some studies noting changes in both enzymatic antioxidants, such as superoxide dismutase and catalase, and more reliably nonenzymatic markers such as glutathione. In addition, studies in animal models have reported neuropathological changes in response to hexavalent chromium in multiple studies across multiple species with effects reported in the cerebellum and effects that have included hemorrhage, neuronal vacuolation, and edema. Additional studies report its accumulation in the hippocampus and ability to produce metal dyshomeostasis in the brain (Vielee et al., 2024).

Lead is a well-recognized neurotoxicant experienced via several routes, including industrial pollution, consumer products, and occupational exposures. Because this heavy metal can cross the blood–brain barrier (Bressler and Goldstein, 1991; Garza et al., 2006), it is consistently found to adversely impact the brain. Lead can mimic calcium ions and induce neurotransmitters on nerve endings to release, preventing normal synapsis (Bressler and Goldstein, 1991). Lead can directly induce neuronal apoptosis along with damaging mitochondria, which can also lead to cellular death (Lidsky and Schneider, 2003). Oxidative stress is another well-documented mechanism for lead neurotoxicity through disrupting neuronal homeostasis and leading to neuroinflammation (Lopes et al., 2016). Furthermore, because it can mimic calcium and block calcium channels, it bioaccumulates in bone and can recirculate later in life as bone demineralizes with age, which can induce neurologic responses long after initial lead exposures occurred (Oliveira et al., 2002).

Exposure Timing, Critical Windows, and Cumulative Exposure

When considering the exposure-disease paradigm for toxicants and mental and behavioral health outcomes, it is critical to consider exposure timing, including whether the exposure is acute or chronic. In the case of the committee’s study population, several military exposures, especially those occurring in residential dwelling or during typical occupational tasks, can be chronic. However, the committee’s analysis does not account for early-life exposures in childhood and adolescence, which are vulnerable windows for neurodevelopment and may influence risk of mental health presentation in adulthood. The committee also did not have data on postdeployment exposures. Additionally, some conditions require a long latency period between exposure occurrence and clinical disease onset, particularly for neurodegenerative disorders such as amyotrophic lateral sclerosis, Parkinson’s disease, and dementia. Environmental exposures are often time-varying. However, the committee was unable to consider this key component in understanding the exposure–disease relationships examined given the data provided. The committee further recognizes that military deployment may not be the relevant window of exposure for the associations investigated. A life-course approach, examining exposures from birth through late-life, with longitudinal data would better elucidate the associations of interest.

Military personnel are more likely to face multiple instances of exposure and different mixtures of exposures. This raises the possibility of interactive and cumulative effects and enhanced toxicity, so evaluating the potential impacts of cumulative exposure on adverse health outcomes warrants special consideration. Cumulative exposure to a single pollutant would require consideration on duration, frequency, and concentration. To date, studies of such possibilities in relation to military exposures are not reported. Cumulative exposure to a mixture of pollutants additionally requires a well-defined mixtures-related question, such as about the (1) overall effect of the mixture (i.e., as all pollutants in the mixture increase in concentration, does the health response also increase?); (2) identification of toxic agent(s) within the mixture (i.e., which pollutant(s) are contributing most to the effect?); (3) potential interactions among pollutants to elucidate potential synergistic or antagonistic effects (i.e., when chemicals affect health via the same mechanism, like neuroinflammation or oxidative stress, “dose-additivity” may occur—exposure to one chemical is not sufficient to produce effects, but multiple exposures acting via these mechanisms ultimately overcome any physiological compensation to induce toxicity); or (4) specific patterns of exposure in a population that could, for example, represent different sources or behaviors that gave rise to the exposure levels (Gibson et al., 2019; Joubert et al., 2022). Different statistical methods are appropriate, as no statistical method outperforms others for all mixtures-related questions (Joubert et al., 2025; Taylor et al., 2016).

Furthermore, integrative physiology must be considered as well. Different exposures may act via different molecular or cellular mechanisms but produce a common downstream effect. Such convergent effects have been shown, for example, in reductions in levels of androgenic hormones. Studies that have included exposures to a mixture of 15 different pesticides and phthalates, each known individually to produce male reproductive tract defects by reducing androgen, when administered together at doses of each that by itself were without effect nevertheless led to toxicity (Conley et al., 2021). Such studies demonstrate the potential for cumulative toxicity even though individual chemicals were without effect and acted via different molecular initiating events that ultimately yielded to reduced levels of androgenic hormones and male reproductive tract defects. This raises the possibility that assessments of associations of individual chemical exposures with mental, behavioral, or neurologic health outcomes may be null, whereas assessments of cumulative exposures could reveal different consequences.

SUMMARY

This chapter discusses the military-related environmental and occupational exposures available in the data provided to the committee and chosen for its analyses of possible relationships among the mental, behavioral, and neurologic outcomes of interest and such exposures: burn pits; dust and particulates (road dust, desert dust, PM₁₀, PM_2.5); exhaust (jet and diesel); fuels (jet and diesel); incinerator emissions; metals (aluminum production/smelting, lead, hexavalent chromium, welding, depleted uranium); mold; radiation (ionizing radiation and nickel-63); and solvents. For each one, the chapter describes how a service member may have been exposed during deployment, whether through the environment, specific occupations during deployment, or both. Biologic plausibility of the exposures potentially affecting risk of the outcomes of interest, broad mechanisms of action (oxidative stress, neuroinflammation, and neurotoxicity), and cumulative exposure to a single or multiple toxicants are also briefly discussed as relevant factors in analyzing the potential exposure–outcome relationships.

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