Rates of loss and replacement of very small particles (VSP) on the contact surfaces of footwear during successive exposures.

It has previously been shown that a walk of 250 m (approximately 175 steps/shoe) in a new environment is sufficient to remove and replace particles present on the contact surfaces of footwear. However, it is unknown how quickly this replacement occurs. This paper describes experiments measuring how quickly (in terms of steps) this loss and replacement of VSP occurs. Three environmental exposure sites were chosen to have different, characteristic particle types (soil minerals). Footwear of two types (work boots and tennis shoes) were tested, accumulating particles by walking 250 m in one site, followed by walking a designated number of steps (per shoe) in a second site. Very small particles (VSP) were harvested from contact surfaces of the footwear (those surfaces in direct contact with the ground when walking) using a moist swabbing procedure. The resulting numbers and types of VSP were determined using forensic microscopy and the proportions of VSP attributable to the first and second site were determined by particle combination analysis using a latent Dirichlet allocation model. The principal findings from this study are (1) that the contact surfaces of footwear are dominated by VSP attributable to the most recent site of exposure, (2) that walking in a new location rapidly removes and replaces VSP from the contact surfaces of footwear, (3) major replacement occurs in 5 to 10 steps and nearly complete replacement occurs by 25 steps, (4) the character of the loading site may influence the initial rate of loss and replacement of VSP (during the first five steps), and (5) overall, the loss and replacement of VSP on footwear in these experiments can be reasonably described by the relationship 1/(Steps + 2.68). The significance of these findings is considerable. In cases where the last site visited is of interest, VSP from contact surfaces of footwear will give a nearly pure sample of that site. In cases where prior sites visited are of interest, the VSP from the contact surfaces can be used as a background signal, providing a necessary beginning for efforts to resolve mixtures of VSP arising from different exposures. The usefulness of VSP on footwear evidence depends on developing this ability so that a reliable, relevant evidentiary "signal" can be separated from background noise (or signals from other exposures).