# Airborne and Dermal Collection Methods of Gunshot Residue for Toxicity Studies
## 1 Introduction  
Recreational and professional use of firearms is common throughout the world, with an estimated 11.5 million people in the United States participating in recreational hunting and 32 million firearm users [1]. Gunshot residue (GSR) is the organic and inorganic components deposited in the air and surrounding environment after the discharge of a firearm [2]. Common elements considered tracers of GSR include lead (Pb), barium (Ba), and antimony (Sb). Other elements found in GSR include aluminum (Al), sulfur (S), calcium (Ca), zinc (Zn), copper (Cu), titanium (Ti), and silicon (Si) [3]. Particles of various sizes are present in GSR following the combustion of the primer and fractionation of the projectile or barrel of the firearm [4]. In addition to elemental components of GSR, recent work has focused on the detection and analysis of organic GSR to establish a more complete chemical profile [5, 6, 7, 8]. Traditionally, the study of GSR has been related to applications for crime investigation. This research has included determining the air suspension time and deposition area of GSR in indoor, controlled environments [9, 10, 11]. Previous investigations primarily used carbon-based adhesives on stubs [4, 12, 13], but other methods look at nasal mucus [14, 15, 16]. Different instrumentation coupled with mass spectrometry have been used to confirm GSR [7, 17, 18, 19]. While this previous literature has been useful for forensic purposes, there is a need to understand the behavior of GSR particles in outdoor settings as well as improve the understanding of the human exposure and environmental implications of the discharge of firearms [20]. Research into the health effects of GSR is a growing area of interest with firearm at military firing ranges [21; 22]. Some of the common elements found in GSR have known health relevance such as Pb [23; 24; 25] and Ba [26; 27]. Recent studies investigated the in vitro toxicity of GSR [28] and through use of a lung deposition model [29]. Oxidative stress and inflammation were increased following exposure to GSR in two epithelial airway cell lines [28]. Another area of concern related to GSR is the environmental impact as the dispersion of different sizes of GSR particles may vary. The current literature available focuses on the impact of Pb deposited into the soil through bullet fragments [30; 31; 32], but a subset of studies detected increased concentration of metals in the soil near the firing line of different ranges [33; 34], presumably from the settling of airborne GSR particles. Combined, these studies show the necessity to continue researching the environmental and human health impacts of GSR. Notably, the available research is limited to a few studies analyzing GSR particles in an outdoor setting [29] and detecting fine particulates that would be respirable to the shooter and bystanders [28; 29; 35]. The determination of size deposition of GSR is important to understand due to the health effects related to different fractions. Fine particulate matter (PM\({}_{2.5}\)) can directly enter the lungs due to the small size of the particles, being less than 2.5 microns in aerodynamic diameter [36]. Particle size distribution of organic and inorganic components from GSR is a key factor in the determination of health and environmental effects related to PM\({}_{2.5}\). A limited number of studies observed the deposition and particle size in an indoor or controlled setting [10; 11; 28; 29; 35; 37] but there is a lack of research, particularly in outdoor settings, for characterizing PM\({}_{2.5}\) from GSR. PM\({}_{2.5}\) is composed of a number of health-relevant components: including the aforementioned inorganic and organic species in GSR as well as black carbon which has known health effects [38; 39]. Black carbon is a component of PM\({}_{2.5}\) that is emitted during combustion reactions from a number of sources such as biomass burning, fuel exhaust, and industry [38; 40].