Author: William M. Davis
Because studies have shown varying amounts of gunshot residue (GSR) in the police environment, with most being on surfaces within facilities occupied and operated by law enforcement, the current study examined whether GSR contamination could be found on the hands of detainees at the Harris County Jail (Texas) who were not charged with any offenses related to the discharge of a weapon.
Of the 175 samples obtained and tested, not one characteristic GSR particle was found. Coupling the results of this study with one in which no GSR particles were on 100 persons answering bench warrants in Bexar County (Texas) gives an average of less than one GSR particle in 275 hand samples. Considering that a different study of random surfaces within the Chicago Police Department (n=201) found a total of 56 GSR particles, the current study concludes that GSR particles are not readily transferred to surfaces not in close proximity to a recently fired gun.
Applying the Poisson (counting) probability model to the current combination of findings led to the probabilities that arise for the detection of small numbers of GSR particles among populations inhabiting facilities operated by law enforcement officers. The study recommends that a best-practice for GSR analysis could be a threshold of three characteristic Pb/Ba/Sb particles, since the presence of this number of particles is unlikely to be found as the result of chance exposure. A higher threshold lessens the probability further.