Gunshot residue background on police officers: Considerations for secondary transfer in GSR evidence evaluation.

In order to have confidence in the integrity of a GSR test result, it is essential to understand the factors that may influence the presence of GSR particles on a person of interest's hands beyond the discharge of a firearm. One important consideration is the possibility that a person of interest (POI) may become contaminated with GSR through their interactions with firearms-carrying police officers during the process of apprehension or arrest. This situation has the potential to result in a false positive error, and potentially result in the mistaken inclusion of a POI in an investigation. Prior studies have indicated that receiving a firearm at the start of shift represents a significant contamination event, which may result in GSR persisting on the hands of police officers (Cook, 2016). Firearms-carrying police officers were sampled at various points throughout their shift to ascertain the extent of persistence of GSR on their hands, as a potential population that could then be transferred. A secondary transfer study was also conducted, with several 'mock arrest' scenarios used as a means of modelling the extent and dynamics of any secondary GSR transfer. When compared to the random population, it was found that police exhibit GSR contamination of their hands at a higher frequency. 7.9% of the officers sampled in this study returned at least one, 3-component particle characteristic of GSR. Further, 75% of the officers tested had at least one particle considered consistent with GSR present on their hands. However, the overall particle numbers were relatively low, with the average reported number of characteristic particles being less than five, and the greatest number reported being twelve. When considering the possibility of secondary transfer occurring during the process of arrest, it was further determined through secondary transfer experiments that the most probable transfer situation was fewer than 25% of the particles present on an officer's hands would be transferred to a POI as a result of a short 'mock arrest' contact. We conclude that although the possibility of GSR contamination through the process of arrest exists, and is worthy of consideration in the assessment of GSR evidence, it is not an overwhelming, or major concern.