Contributions of particulate and gas phases of simulated burn pit smoke exposures to impairment of respiratory function

Current understanding of the impact of United States military airborne hazards and burn pit exposures on respiratory health

Millions of United States (U.S.) troops deployed to the Middle East and Southwest Asia were exposed to toxic airborne hazards and/or open-air burn pits. Burn pit emissions contain particulate matter combined with toxic gasses and heavy metals. Ongoing research has demonstrated that exposures to the airborne hazards from military burn pits have profound and lasting health and wellness consequences. Research on the long-term health consequences of exposure to open burn pits has been limited. Work continues to understand the scope of the health impacts and the underlying pathobiology following exposures and to establish care standards. The U.S. Sergeant First Class Heath Robinson Honoring our Promise to Address Comprehensive Toxics (PACT) Act was signed into law August 2022. This act expands the benefits and services to U.S. Veterans exposed to toxicants, requires the Veterans Health Administration to provide toxic exposure screening, and supports increased research, education, and treatment due to toxic occupational exposures. This review highlights the state of the science related to military burn pit exposures research with an emphasis on pulmonary health. Clinical data demonstrate areas of reduced or delayed pulmonary ventilation and lung pathologies such as small airways scarring, diffuse collagen deposition and focal areas of ossification. Identification and characterization of foreign matter deposition in lung tissues are reported, including particulate matter, silica, titanium oxides, and polycyclic aromatic hydrocarbons. These data are consistent with toxic exposures and with the symptoms reported by post-deployment Veterans despite near-normal non-invasive pulmonary evaluations. On-going work toward new methods for non-invasive pulmonary diagnoses and disease monitoring are described. We propose various studies and databases as resources for clinical and health outcomes research. Pre-clinical research using different burn pit modeling approaches are summarized, including oropharyngeal aspiration, intranasal inhalation, and whole-body exposure chamber inhalation. These studies focus on the impacts of specific toxic substances as well as the effects of short-term and sustained insults over time on the pulmonary systems.

Effects of simulated smoke condensate generated from combustion of selected military burn pit contents on human airway epithelial cells

BACKGROUND

Exposure to military burn pit smoke during deployment is associated with different respiratory and non-respiratory diseases. However, information linking smoke exposure to human pulmonary health is lacking. This study examined the effects of simulated burn pit smoke condensates on human airway epithelial cells (HAECs) from twelve donors (smokers/non-smokers, biological female/male) cultured at an air-liquid interface and exposed to condensates from three simulated burn pit waste materials (cardboard, plywood, and plastic) incinerated at two combustion conditions: smoldering and flaming. Cellular gene expression was analyzed using bulk RNA sequencing, and basolateral media cytokine levels were assessed using multiplex immunoassay.

RESULTS

Flaming smoke condensates caused more significant differentially expressed genes (DEGs) with plywood flaming smoke being the most potent in altering gene expression and modulating cytokine release. Cardboard and plywood flaming condensates primarily activated detoxification pathways, whereas plastic flaming affected genes related to anti-microbial and inflammatory responses. Correlation analysis between smoke condensate chemicals and gene expression to understand the underlying mechanism revealed crucial role of oxygenated polycyclic aromatic hydrocarbons (PAHs) and aluminum, molybdenum, and silicon elements; IL6 expression was positively correlated with most PAHs. Stratification of data based on HAEC donor demographics suggests that these affect gene expression changes. Enrichment analysis indicated similarity with several deployment-related presumptive and reported diseases, including asthma, emphysema, and cancer of different organs.

CONCLUSIONS

This study highlights that simulated burn pit smoke exposure of HAECs causes gene expression changes indicative of deployment-related diseases with more pronounced effects seen in smokers and females. Future studies are needed to further characterize how sex and smoking status affect deployment-related diseases.

Burn Pit Smoke Condensate-Mediated Toxicity in Human Airway Epithelial Cells

Burn pits are a method of open-air waste management that was common during military operations in Iraq, Afghanistan, and other regions in Southwest Asia. Veterans returning from deployment have reported respiratory symptoms, potentially from exposure to burn pit smoke, yet comprehensive assessment of such exposure on pulmonary health is lacking. We have previously shown that exposure to condensates from burn pit smoke emissions causes inflammation and cytotoxicity in mice. In this study, we explored the effects of burn pit smoke condensates on human airway epithelial cells (HAECs) to understand their impact on cellular targets in the human lung. HAECs were cultured at the air-liquid interface (ALI) and exposed to burn pit waste smoke condensates (plywood, cardboard, plastic, mixed, and mixed with diesel) generated under smoldering and flaming conditions. Cytotoxicity was evaluated by measuring transepithelial electrical resistance (TEER) and lactate dehydrogenase (LDH) release; toxicity scores (TSs) were quantified for each exposure. Pro-inflammatory cytokine release and modulation of gene expression were examined for cardboard and plastic condensate exposures. Burn pit smoke condensates generated under flaming conditions affected cell viability, with flaming mixed waste and plywood exhibiting the highest toxicity scores. Cardboard and plastic smoke condensates modulated cytokine secretion, with GM-CSF and IL-1β altered in more than one exposure group. Gene expression of detoxifying enzymes (ALDH1A3, ALDH3A1, CYP1A1, CYP1B1, NQO1, etc.), mucins (MUC5AC, MUC5B), and cytokines was affected by several smoke condensates. Particularly, expression of IL6 was elevated following exposure to all burn pit smoke condensates, and polycyclic aromatic hydrocarbon acenaphthene was positively associated with the IL-6 level in the basolateral media of HAECs. These observations demonstrate that exposure to smoke condensates of materials present in burn pits adversely affects HAECs and that aberrant cytokine secretion and altered gene expression profiles following burn pit material smoke exposure could contribute to the development of airway disease.

Burn pit-related smoke causes developmental and behavioral toxicity in zebrafish: Influence of material type and emissions chemistry

Combustion of mixed materials during open air burning of refuse or structural fires in the wildland urban interface produces emissions that worsen air quality, contaminate rivers and streams, and cause poor health outcomes including developmental effects. The zebrafish, a freshwater fish, is a useful model for quickly screening the toxicological and developmental effects of agents in such species and elicits biological responses that are often analogous and predictive of responses in mammals. The purpose of this study was to compare the developmental toxicity of smoke derived from the burning of 5 different burn pit-related material types (plywood, cardboard, plastic, a mixture of the three, and the mixture plus diesel fuel as an accelerant) in zebrafish larvae. Larvae were exposed to organic extracts of increasing concentrations of each smoke 6-to-8-hr post fertilization and assessed for morphological and behavioral toxicity at 5 days post fertilization. To examine chemical and biological determinants of toxicity, responses were related to emissions concentrations of polycyclic hydrocarbons (PAH). Emissions from plastic and the mixture containing plastic caused the most pronounced developmental effects, including mortality, impaired swim bladder inflation, pericardial edema, spinal curvature, tail kinks, and/or craniofacial deformities, although all extracts caused concentration-dependent effects. Plywood, by contrast, altered locomotor responsiveness to light changes to the greatest extent. Some morphological and behavioral responses correlated strongly with smoke extract levels of PAHs including 9-fluorenone. Overall, the findings suggest that material type and emissions chemistry impact the severity of zebrafish developmental toxicity responses to burn pit-related smoke.

Military burn pit exposure and airway disease: Implications for our Veteran population

Millions of veterans have been exposed to burn pit smoke during combat deployments throughout the last three decades. Toxic compounds present in burn pit fumes that may cause or exacerbate upper and lower airway diseases include dioxins, polyaromatic hydrocarbons, and particulate matter, among others. There have been several observational studies evaluating the potential role of burn pit exposure in the development of a multitude chronic health conditions, and the veterans Administration has established the Airborne Hazards and Open Burn Pit Registry in 2014. However, specific causality of airway disease from burn pits has been difficult to prove, and there are multiple barriers toward etiologic research. Preclinical models have demonstrated airway dysfunction and inflammation but modeling human exposures remains challenging. Here, we review the current literature on the potential impact of burn pit exposure on chronic airway disease.