5. Are known pathogens confirmed to be present in the lungs of deployers with the condition of interest?
1 PECOT QUESTION
Are known pathogens known to be present in the SWATO confirmed to be present in the lungs of deployers with the condition of interest?
Why is this question important?
If infiltrates related to development or worsening of interstitial lung diseases (ILD) are known to be present in the Southwest Asia Theater of Operations (SWATO) and in the lungs of deployed personnel, then it strengthens the inference that airborne hazards encountered during military personnels’ time in the SWATO contribute to the development of ILD among this population.
2 CONCLUSION
Based on limited and emerging evidence, deployers appear to have a greater amount of particulate matter (PM) in the lungs than non deployed military, and deployers appear to have higher levels of silica and silicates (and perhaps titanium and titanium oxides) in their lungs than control subjects. The presence of these substances is known to be associated with inflammation and fibrosis. While the studies that provided the primary evidence for this question were low risk of bias, the limited number of studies and small samples qualifies our confidence. However, there are no serious alternative explanations.
Evidence Level: Considering evidence for the primary hypotheses, combined with uncertainty and evidence for the alternative hypotheses, the evidence level for this conclusion is Low/Moderate, while the evidence level for the alternative explanations is Low.
Figure 1 . Evidence Level
3 KNOWN BIASES
There are known biases in the research on military exposures. We summarize the direction and magnitude of these biases in the table below.
Table 1 . Known Biases: Direction and Magnitude
In summary, for the case reports included in this analysis (n=4 studies), we cannot generalize to other deployers. In the one available cohort study, we would expect higher levels of several types of PM in controls. For the analyses relating PM to exposure within the deployers, there is likely an overestimate of the exposure due to bias from self-report.
4 SUMMARY
The research on types of PM in the lungs of military personnel deployed to SWATO is extremely limited, with only one recent cohort study comparing the levels of different types of PM to control subjects. 3 Of the remaining studies, four are case reports 7-10 and so do not tell us about the levels in lung infiltrates in deployers in general. Three case-series were identified. Huang et al 2019 11 provides a case-series comparison between patients diagnosed with IPF and n=2 deployers. Another case series is nested within a cohort study and, while it provides some information on infiltrates, there is no comparison to non-deployed groups 12 . Finally, Strausborger et al 2021 13 examines six types of PM in the lungs of deployers and non-deployed military, but (because of the small sample size) do not carry out a statistical comparison. Thus, the strongest current evidence is available from the Lowers et al 2024 analysis. (Note, an article detailing novel methods for detecting PM in lungs was recently published, and draws on a comparative sample of deployers and non-deployed military. 14 However, authors state that the study detailing the comparison between deployed and non-deployed will be forthcoming, and so was not available for this synthesis.)
Across the case reports and case series studies, the most commonly identified infiltrates are titanium and iron ( Table 1 ). In these particular deployers, the authors make a connection between the presence of these infiltrates and lung scarring, fibrosis and disease. The mechanistic basis for this link is well established with different forms of iron and titanium and other metals in the lung found to be associated with lung inflammation and remodeling of lung tissue, contributing to both fibrosis and granuloma formation. 5,6,15,16 Similarly, in their experiments exposing animals and human tissue to dust from the SWATO (where iron and titanium are known to be present in the PM of this area 17,18 ), Berman and colleagues demonstrate the production of reactive oxygenation species (ROS) and resulting inflammation in the lung tissues. 19
Table 2 . Lung Infiltrates Identified in Case Reports and Case Series
However, as noted above, while we can have confidence that these infiltrates were found within these cases, and that the presence of these metal containing particulates are associated with common features of interstitial lung diseases, we do not know whether the types and amounts of infiltrates differ between deployed and non-deployed. Being able to distinguish particular types of infiltrates sourced from the SWATO but rare in US-stationed military is crucial.
Regarding amount of particulate matter in the lung, however, we do have some initial evidence from Strausborger et al 2021 that military stationed in Afghanistan, Iraq or both have a greater particulate load than non-deployed military. Though their article was largely methodological (more of a feasibility study) and did not carry out statistical comparisons due to small sample sizes, they do report the following differences ( Figure 1 ).
Figure 2 . Amount of PM in the Lung by Time in Theater
Lowers et al 2024, the only cohort study, used scanning electron microscopy imaging to identify eight different categories of PM, comparing deployers to samples from accident victims with no known history of lung disease. Even with small and imbalanced samples, they identified significant differences between deployers and controls in terms of the type of PM in the lung ( Figure 2 ). Thus, while silica and silicate particles are common in urban adults 13 , these substances were substantially and significantly higher in deployers than controls, which is the opposite of what we would expect based on the higher level of smoking and greater age of the controls. Significantly, inhaled silica/silicates and titanium are associated with the progression of idiopathic pulmonary fibrosis and higher levels are associated with worse prognosis, even after adjusting for smoking. 20
Figure 3 . Difference between Deployers and Controls in Terms of Type of PM in the Lung
It is unsurprising that the control subjects have higher levels of carbonaceous particles given that controls were significantly more likely to smoke and had a greater number of pack years compared to deployers (see Overview Table). Among different types of airborne hazards (e.g., smoke from fires, air pollution, diesel exhaust, etc.) smoking produces much higher levels of carbonaceous particles in the lungs. 4
Lowers et al also seek to link lung PM characteristics to self-reported exposure (e.g., sandstorms, burn pits, total PM) and military occupation. Due, in part, to the small sample, and also due to self-report bias (i.e., misremembering how often one was exposed to sandstorms, etc.), no statistically significant differences were identified with exposure measures and effect sizes were very small. For military occupation (notably, the only non-self-report exposure characteristic), compared to non-combat military occupations, those in combat positions had a 36% higher total PM burden (95% CI: -10%, 103%). While this comparison did not reach statistical significance, the distribution of the confidence interval suggests that a larger study may substantiate these initial findings.
Technologies for identifying PM in lungs is advancing 14,21 and recent methodological developments will allow for faster and more accurate comparisons in the future. At this time, however, there are still a limited number of studies, none of which directly compare deployers to non-deployed military—though this is anticipated. 14
In Summary: In part, because of previous technological limitations, larger studies comparing deployers to non-deployers are limited. However, based on the evidence that is available, deployers appear to have a greater amount of PM in the lungs than non-deployed military, and deployers appear to have higher levels of silica and silicates (and perhaps titanium and titanium oxides) in their lungs than control subjects. The presence of these substances is known to be associated with inflammation and fibrosis.
5 UNCERTAINTY
Early Research: The research on lung infiltrates in deployers is still in its infancy. This is substantiated by the limited number of comparative studies combined with the methodological papers published in recent years 14,21 . Thus, while we have a comparison between deployers and civilian controls, we do not yet have a direct comparison between deployers and non-deployed military large enough for a statistical comparison (though, this research is forthcoming) 14 . We know that even non-deployed military personnel are exposed to occupational airborne hazards. 22 So, it is possible that the differences identified between deployers and controls would be somewhat reduced in a deployer versus non- deployer comparison.
6 STRENGTH OF EVIDENCE
Following recent guidance regarding the baseline evidence level for study designs of this type used in complex environmental exposure research 23,24 , studies start at a moderate confidence level. Then, following the GRADE approach of rating up or rating down based on characteristics of the body of research, we reach a final strength of evidence rating for each of the three related PECOT questions.
While there were eight studies that addressed this question, the four case reports were disregarded when drawing conclusions regarding the comparison of infiltrates in the lungs of deployers versus non-deployers. Thus, the overall risk of bias estimate is based only on these two studies (Strausborger et al 2021 and Lowers et al 2024). For the purpose of characterizing lung infiltrates we deem both cohort and comparative case-series designs to have a baseline Moderate Confidence level. 23,24 The residual confounding decision is based on the disproportionate number of smokers and older individuals in the control arm (mean pack years in deployers = 5.0 ± 4.5 versus 12.2 ± 19.8 in controls; controls nearly 14 years older on average). Both of these characteristics would lead us to expect to find higher levels of carbonaceous, silica/silicates and titanium in the lungs of controls. While this proved true for carbonaceous infiltrates (due to smoking), the opposite effect was seen with silica/silicates (which we would expect to be higher due to age) and titanium (due to smoking). Thus, not only were levels of silica/silicates and titanium higher in the deployers, this is opposite of what we would expect given the demographic imbalances in this sample. This makes it more likely that the effects reported in Lowers et al are true. However, since Lowers and colleagues compare PM levels to controls, rather than non-deployed military, there are some minor concerns regarding indirectness.
7 ALTERNATIVE EXPLANATIONS
The most plausible alternative explanations for differences in the histomorphology between deployed versus non-deployed military are:
- Differences in prevalence of smoking (there is evidence that deployers are more likely to smoke
than non-deployers)
The PM footprint of cigarette smoking is largely carbonaceous and is a product of an incomplete combustion 4 , though titanium containing particles are released in cigarette smoke (and from the filter tip of cigarettes) 25 . So, even if the military personnel in the above analyses were smokers, it would be clear which of the PM types were associated with smoking. Additionally, in Lowers et al 2024, the deployers were significantly less likely to smoke than controls. Thus, the explanation for higher levels of silica, silicates and titanium cannot be from cigarette smoking.
Regarding explanation due to smoking: In the studies examined, the preponderance of silica, silicates and titanium in the lungs of deployers cannot be explained by smoking alone.
- Differences in occupational or environmental exposures following separation from military servicePM (especially silica and silicates) is present in the lungs of virtually all urban dwelling adults. 13 Thus, we would expect some proportion of the presence of these infiltrates to be a result of non- deployment exposures. However, the comparison to civilian controls reveals significantly higher levels in deployers, which cannot be explained by known environmental exposures.
Regarding differences in occupational and environmental exposures, there is no available evidence to suggest that, following service, formerly deployed versus non-deployed veterans have differential exposure to pathogens shown to be associated with ILD.
8 OVERVIEW TABLE
9 RISK OF BIAS
The risk of bias assessment for the included articles is presented below for each type of study design (see Figure 3 and Figure 4 ).
9.1 COHORT DESIGN
Figure 4 . Cohort Design Risk of Bias
9.2 CASE-SERIES OR CASE REPORT DESIGN
Figure 5 . Case-Series or Case Report Design Risk of Bias
In general, all case reports suffered from multiple risks. In part, this is due to the nature of the design, but, additionally, most were conference abstracts and so provided insufficient detail. The two case-series designs (Olsen et al 2022 and Strausborger et al 2021) and the cohort design (Lowers et al 2024) were low risk.
Given that the primary findings for this question are drawn only from the two comparative studies (Strausborger et al 2021 and Lowers et al 2024), the overall risk of bias for this question is low.
10 METHODOLOGICAL NOTE
In addition to the limited number of studies, the two studies that provide the primary evidence for this question are underpowered. Because of the small n samples, Strausborger et al do not attempt statistical comparisons. In Lowers et al, the combined small samples and imbalance between arms decreases power. This is evident in the extremely wide confidence intervals.
11 REFERENCES
1. Obernolte H, Niehof M, Braubach P, et al. Cigarette smoke alters inflammatory genes and the extracellular matrix—investigations on viable sections of peripheral human lungs. Cell and Tissue Research. 2022:1-12.
2. Harte CB, Proctor SP, Vasterling JJ. Prospective Examination of Cigarette Smoking Among Iraq- Deployed and Nondeployed Soldiers: Prevalence and Predictive Characteristics. Annals of Behavioral Medicine. 2014;48(1):38-49. doi:10.1007/s12160-013-9584-5
3. Lowers H, Zell-Baran L, Arslan Z, Moore CM, Rose C. Particle Morphology and Elemental Analysis of Lung Tissue from Post-9/11 Military Personnel with Biopsy-Proven Lung Disease. International Journal of Environmental Research and Public Health. 2024;21(1):91.
4. Sangani RG, Ghio AJ. Lung injury after cigarette smoking is particle related. Int J Chron Obstruct Pulmon Dis. 2011;6:191-8. doi:10.2147/COPD.S14911
5. O’Day PA, Pattammattel A, Aronstein P, Leppert VJ, Forman HJ. Iron Speciation in Respirable Particulate Matter and Implications for Human Health. Environ Sci Technol. Jun 7 2022;56(11):7006- 7016. doi:10.1021/acs.est.1c06962
6. Oliveira VR, Uriarte JJ, Falcones B, et al. Biomechanical Response of Lung Epithelial Cells to Iron Oxide and Titanium Dioxide Nanoparticles. Front Physiol. 2019;10:1047. doi:10.3389/fphys.2019.01047
7. Dodson D, Kwon HP, Zacher L. Gold and Teflon from transbronchial biopsies of a combat veteran with shortness of breath after exposure to burn pits in Iraq. presented at: Chest; 2010;
8. Rossiello M, Szema A, Venuto D, et al. Titanium Lung in a Solider with Iraq/Afghanistan War- Lung Injury (IAW-LI). A49 CASE REPORTS IN LUNG DISEASE ASSOCIATED WITH INHALATIONAL EXPOSURES. American Thoracic Society; 2019:A1818-A1818.
9. Wolff J, Krefft S, Rose CS. Chronic Eosinophilic Pneumonia and Asthma Following Military Deployment. presented at: American Thoracic Society International Conference; 2019; Dallas, TX. Session CASE REPORTS INLUNG DISEASE ASSOCIATED WITH INHALATIONAL EXPOSURES.
10. Szema AM, Schmidt MP, Lanzirotti A, et al. Titanium and iron in lung of a soldier with nonspecific interstitial pneumonitis and bronchiolitis after returning from Iraq. J Occup Environ Med. Jan 2012;54(1):1-2. doi:10.1097/JOM.0b013e31824327ca
11. Huang P, Szema A, Venuto D, et al. Titanium Is Abundant in Fibrotic Lungs of Two Soldiers Post- Deployment to Iraq and Afghanistan but Is Not Seen in Lungs of Five Patients with Idiopathic Pulmonary Fibrosis. presented at: A52 SARCOIDOSIS AND EXPOSURE RELATED INTERSTITIAL LUNG DISEASE; 2019;
12. Olsen T, Caruana D, Cheslack-Postava K, et al. Iraq/Afghanistan war lung injury reflects burn pits exposure. Scientific Reports. 2022;12(1):14671.
13. Strausborger SL, Jenkins HM, Franks TJ, Lewin-Smith MR. Particle Analysis by Scanning Electron Microscopy With Energy Dispersive X-Ray Analysis in Pulmonary Pathology Specimens From U.S. Military Service Members Deployed During the Global War on Terror 2002 to 2015. Mil Med. Jan 25 2021;186(Suppl 1):784-788. doi:10.1093/milmed/usaa318
14. Hayden L, Strausborger S, Lewin-Smith M. Automated Particle Analysis Using Field-Emission Scanning Electron Microscopy (FE-SEM) and Energy Dispersive X-Ray Spectroscopy (EDS) to Characterize Inhaled Particulate Matter (PM) in Biopsied Lung Tissue. Microscopy and Microanalysis. 2023;29(1):235- 243. doi:10.1093/micmic/ozac015
15. Assad N, Sood A, Campen MJ, Zychowski KE. Metal-Induced Pulmonary Fibrosis. Curr Environ Health Rep. Dec 2018;5(4):486-498. doi:10.1007/s40572-018-0219-7
16. Shamsollahi HR, Jahanbin B, Rafieian S, Yunesian M. Particulates induced lung inflammation and its consequences in the development of restrictive and obstructive lung diseases: a systematic review. Environmental Science and Pollution Research. 2021;28:25035-25050.
17. Engelbrecht JP, McDonald EV, Gillies JA, Jayanty RK, Casuccio G, Gertler AW. Characterizing mineral dusts and other aerosols from the Middle East–Part 2: grab samples and re-suspensions. Inhal Toxicol. Feb 2009;21(4):327-36. doi:10.1080/08958370802464299
18. Engelbrecht JP, McDonald EV, Gillies JA, Jayanty RK, Casuccio G, Gertler AW. Characterizing mineral dusts and other aerosols from the Middle East–Part 1: ambient sampling. Inhal Toxicol. Feb 2009;21(4):297-326. doi:10.1080/08958370802464273
19. Berman R, Rose CS, Downey GP, Day BJ, Chu HW. Role of particulate matter from Afghanistan and Iraq in deployment-related lung disease. Chemical Research in Toxicology. 2021;34(12):2408-2423.
20. Koga Y, Satoh T, Kaira K, et al. Progression of idiopathic pulmonary fibrosis is associated with silica/silicate inhalation. Environmental Science & Technology Letters. 2021;8(10):903-910.
21. Lowers HA, Breit GN, Strand M, et al. Method to characterize inorganic particulates in lung tissue biopsies using field emission scanning electron microscopy. Toxicology mechanisms and methods. 2018;28(7):475-487.
22. Barth SK, Dursa EK, Bossarte R, Schneiderman A. Lifetime Prevalence of Respiratory Diseases and Exposures Among Veterans of Operation Enduring Freedom and Operation Iraqi Freedom Veterans: Results From the National Health Study for a New Generation of U.S. Veterans. J Occup Environ Med. Dec 2016;58(12):1175-1180. doi:10.1097/JOM.0000000000000885
23. Boogaard H, Atkinson RW, Brook JR, et al. Evidence Synthesis of Observational Studies in Environmental Health: Lessons Learned from a Systematic Review on Traffic-Related Air Pollution. Environ Health Perspect. Nov 2023;131(11):115002. doi:10.1289/EHP11532
24. Steenland K, Schubauer-Berigan MK, Vermeulen R, et al. Risk of Bias Assessments and Evidence Syntheses for Observational Epidemiologic Studies of Environmental and Occupational Exposures: Strengths and Limitations. Environ Health Perspect. Sep 2020;128(9):95002. doi:10.1289/EHP6980
25. Fresquez MR, Watson CH, Valentin-Blasini L, Steven Pappas R. Characterizing the Transport of Aluminum-, Silicon- and Titanium Containing Particles and Nanoparticles in Mainstream Tobacco Smoke. J Anal Toxicol. Aug 14 2021;45(7):722-729. doi:10.1093/jat/bkaa162