Particulate Matter from Firearms: Lung Toxicity and Respiratory Effects
Blog post by Nina Culum, MSc
Firing ranges are necessary for law enforcement and military personnel, but are also increasingly used by civilians for recreational purposes, and at least 16,000 exist in the United States alone . While health and safety precautions are rightly focused on the prevention of shooting-related injury and death, heavy metal exposure in these settings also poses surprising and substantial additional health risks [1, 2]. Large amounts of nanoparticles containing heavy metals are likely emitted when firearms are shot, which can reach internal organs upon inhalation and lead to interstitial lung disease, bronchial asthma, and even cardiovascular, neurological, and immunological complications [2, 3].
The size of nanoparticles largely dictates how far they may reach into the respiratory tract and where they are deposited (Figure 1). While most respirable dust (i.e., less than 1 μm in diameter) is exhaled due to low inertia, particles that are less than 500 nm in diameter can be deposited in the lungs because of their diffusional mobility . Additionally, ultrafine particles that are less than 100 nm in size can effectively settle in the alveolar region . In the context of nanoparticle-mediated heavy metal exposure through firearm shooting, lead is of particular concern.
Figure 1: Size-dependent regional deposition of inhaled nanoparticles and how they interact with the lung microbiome. © 2018 Poh et al., licensed under CC BY 4.0.
Despite public health outreach and control efforts, elevated blood lead levels have been reported among employees of indoor firing ranges, their families, and customers . Lesser studied, however, are the toxicological and respiratory effects of other heavy metals emitted during firearm use. Therefore, Kim et al. modeled the acute health effects of particulate matter inhalation from rifles and handguns in mice, with special attention given to the role of copper in lung toxicity . The findings from this study were recently published in Scientific Reports, which we review in this blog post.
Exposing mice to particulate matter from firearm smoke: experimental design
The sampling and preparation of particulate matter from firearm smoke, as well as delivery to mouse lungs, were explained in detail by the authors. Briefly, particulate matter from single shot and burst firings was collected from the muzzle or breech of an M4 rifle, as well as from both the muzzle and breech of a 9-mm handgun following single shot firings. Particulate matter samples in saline solutions were subsequently administered to anesthetized mice by oropharyngeal aspiration, after which respiration and lung toxicity were assessed by whole-body plethysmography and various biochemical assays, respectively. The particulate matter dosage delivered to mice was based on extreme exposure levels at indoor firing ranges. Positive controls received lipopolysaccharide, a well-characterized inflammatory agent, in saline to demonstrate maximal responsiveness, while negative controls received saline only.
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Lung toxicity and respiratory effects of particulate matter: handgun vs. rifle
Prior to investigating the toxicological and respiratory effects of particulate matter from handgun and rifle smoke, the authors physically and chemically characterized these particles to examine how they differ (Figure 2). The majority of particulate matter from both single and burst shots of the rifle was in the 2-3 μm size range, followed by 0.2-0.5 μm, while the opposite was true for particulate matter from handgun smoke. The authors also found that lead was the predominant metal in the particulate matter of handgun smoke (12% by mass), while copper levels were highest in the particulate matter of rifle smoke (26% by mass). In the particulate matter of rifle smoke, most metals existed in water-insoluble form, while only 21% were found to be water-soluble.
Figure 2: Physical (left) and chemical (right) properties of particulate matter from rifle and handgun smoke. © 2022 Kim et al., licensed under CC BY 4.0.
To evaluate the lung toxicity of particulate matter emitted from these firearms, the authors examined mouse lungs for neutrophil accumulation, as well as bronchoalveolar lavage fluid for cellular injury markers and pro-inflammatory cytokines. Interestingly, the particulate matter from rifle smoke, but not handgun smoke, significantly increased neutrophil numbers and concentrations of the pro-inflammatory cytokines interleukin-6 (IL-6) and macrophage inhibitory protein-2 (MIP-2) at 4 and 24 hours following exposure compared to controls. Additionally, rifle and handgun smoke particulate matter both greatly increased the levels of certain cellular injury markers (i.e., protein and microalbumin).
The authors also examined the respiratory effects of particulate matter from rifle and handgun smoke in mice. Ventilatory timing, tidal volume, and other respiratory parameters were significantly increased in mice who were exposed to particulate matter from rifle smoke, but not handgun smoke, at 4 and 24 hours post-exposure, possibly indicating airflow obstruction. The authors also note that these observed changes in lung function were mostly driven by increased expiratory ventilation parameters associated with potential airway narrowing and inflammation.
Toxicological and respiratory effects of copper content and solubility
The authors next focused solely on particulate matter from rifle smoke, since handgun smoke did not significantly affect lung toxicity, and targeted copper and metal solubility for further toxicological testing. Similarly to the particulate matter from rifle smoke, free copper particles significantly increased neutrophil numbers after 4 and 24 hours of exposure. Although copper chelation with penicillamine decreased neutrophil numbers, they remained significantly elevated compared to saline controls. As expected, IL-6 and MIP-2 levels were also significantly higher in mice exposed to copper particles, which was not reduced by copper chelation. Interestingly, copper particle exposure did not affect respiratory parameters, and chelation did not significantly attenuate the respiratory effects of particulate matter from rifle smoke.
While samples containing either water-soluble or -insoluble metals both significantly increased neutrophil numbers at 4 hours post-exposure, only the water-insoluble sample elevated these numbers to a similar degree as the particulate matter from rifle smoke at 24 hours. However, a low-metal sample (i.e., water-soluble sample treated with Chelex) did not significantly increase neutrophil numbers at either time point. Markers of cellular injury were also significantly elevated in mice exposed to the water-insoluble sample, but not water-soluble or low-metal samples. Additionally, IL-6 and MIP-2 concentrations were significantly increased at four hours following exposure to the insoluble sample, similarly to particulate matter from rifle smoke. Lastly, only the water-insoluble sample caused respiratory changes in mice to the same degree as particulate matter from rifle smoke. While the water-soluble sample resulted in smaller respiratory changes, these responses were significantly reduced or even neutralized by chelation.
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What do these findings mean?
Although most ammunition is composed of similar components, their formulations can vary considerably by bullet and firearm type. Depending on the ammunition’s chemical composition and mechanical action of firing, gasses and particulate matter with different physico-chemical properties are emitted, as demonstrated in this study. While particulate matter from handgun smoke did not cause lung toxicity at 24 hours post-exposure, that from rifle smoke did result in detectable toxicity, potentially due to its higher copper levels.
The authors also note that excess copper ions can bind with DNA and affect its structure and integrity, and so the health consequences of chronic exposure to copper from rifle firing should not be ignored. Additional studies are required to identify causal links between the chemical constituents of ammunition and bioactivity, which will ultimately help inform hazard identification, risk assessment, and interventions to reduce exposure to heavy metals when firing.
About the Author
About the Author
Nina Culum graduated from the University of Western Ontario with a Master of Science in physical and analytical chemistry. During her graduate studies, she fabricated plasmonic nanohole arrays to capture extracellular vesicles and detect cancer by surface-enhanced Raman spectroscopy. Prior to attending UWO, Nina completed her Bachelor of Science in chemistry at the University of Waterloo.
- Laidlaw MAS, Filippelli G, Mielke H, Gulson B, Ball AS. Lead exposure at firing ranges—a review. Environ Health. 2017;16:34. DOI: 10.1186/s12940-017-0246-0.
- Lach K, Steer B, Gorbunov B, Mička V, Muir RB. Evaluation of exposure to airborne heavy metals at gun shooting ranges. Ann Occup Hyg. 2015;59(3):307-23. DOI: 10.1093/annhyg/meu097.
- Poh TY, Ali NABM, Aogáin MM, Kathawala MH, Setyawati MI, Ng KW, et al. Inhaled nanomaterials and the respiratory microbiome: clinical, immunological and toxicological perspectives. Part Fibre Toxicol. 2018;15:46. DOI: 10.1186/s12989-018-0282-0.
- Beaucham C, Page E, Alarcon WA, Calvert GM, Methner M, Schoonover TM. Morbidity and mortality weekly report (MMWR): indoor firing ranges and elevated blood lead levels – United States, 2002-2013 [Internet]. Washington, DC: Centers for Disease Control and Prevention; 2014 Apr 25 [cited 2022 Dec 06]. 63(16):347-51. Available from: www.cdc.gov/mmwr/preview/mmwrhtml/mm6316a3.htm.
- Kim YH, Vance SA, Aurell J, Holder AL, Pancras JP, Gullett B, et al. Chemistry and lung toxicity of particulate matter emitted from firearms. Sci Rep. 2022;12:20722. DOI: 10.1038/s41598-022-24856-5.