Specialized Use of Human Scent in Criminal Investigations
Stockham, Rex A., Slavin, Dennis L., Kift, William, Forensic Science Communications
Human-scent evidence is not new. Europeans have been using scent-discriminating canines in criminal investigations for more than 100 years. Recent discoveries and improved techniques, however, have catapulted this tool into the forefront of a number of major criminal investigations. Scent-discriminating dogs have identified the scent of bomb builders from exploded bomb remains, and drive-by shooters have been identified from scent collected off expended cartridge casings.
Using scent-discriminating dogs in criminal investigations should be limited to establishing a scent relationship between people and crime scene evidence. Because human scent is easily transferred from one person or object to another, it should not be used as primary evidence. However, when used in corroboration with other evidence, it has become a proven tool that can establish a connection to the crime.
Human scent has been historically defined as a biological component of decomposing dead skin cells, also known as the skin raft theory (Syrotuck 1972). Scent-dog handlers have relied on this theory, but it has had no supporting scientific basis. Current research suggests that human odor is more complex.
Human Skin Emanation Research for Mosquito Attractiveness
Research has been conducted to determine the components of human odor that is breathed out or is deposited by the skin, not for purposes of specialized canine use, but to identify a mosquito's attraction to human odor. In a study, test subjects transferred their odor by touch onto 2 to 25 2.9mm glass beads. On this small amount of surface area, 346 discernable peaks were detected by cryofocused gas chromatograph/mass spectrometer analyses. All but 43 component peaks were classified and identified (Bernier et al. 2000).
In a study of yellow fever mosquitoes preference for lactic acid, two-choice tests with odor samples from human hands revealed statistically significant differences in the level of mosquito attraction. The differences in mosquito attractiveness between the donors were found to be consistent during the 11-month test period, thus suggesting some level of genetic predisposition to mosquito attractiveness (Steib et al. 2001).
The human odor studies for mosquito attraction show that human scent could consist of many more compounds than can be defined at this time. Not knowing exactly what the dogs are detecting, the identical and fraternal twin studies that explored genetic and dietary differences to determine the uniqueness of scent need to be explored.
The issue of genetic makeup and human odor was studied using scent-discriminating canines and scent collected from three sets of twins. The test subjects included fraternal twin baby boys on the same diet, identical twin baby boys on the same diet, and identical adult twins on different diets. Dogs were able to correctly identify the fraternal baby twins on identical diets and the identical adults twins on different diets, 89 and 83.5 percent respectively. The dogs were able to correctly identify the identical twins on the same diet only 49 percent, which is no better than chance (Hepper 1994).
It was found in scent lineup tests when the scent of identical twins is offered to dogs in succession, the dogs could not differentiate between the twins. In tracking experiments in which two scents are offered simultaneously and mixed up, the dogs were able to distinguish between the two identical twins (Kalmus 1955). These studies show that dogs can differentiate between identical twins, especially when the twins are living apart. However, discrimination is more difficult when the differences were genetic (Schoon and Haak 2002A).
A scent item with multiple human odors is a contaminated-scent article. The term contaminated-scent article, however, is a misleading description. …