Loss and replacement of small particles on the contact surfaces of footwear during successive exposures

- Walking removes and replaces particles on the contact surfaces of footwear.
- Contact surfaces of soles hold particles from the most recent exposure.
- Particles from prior exposures are attributable to recessed areas of footwear soles.
- Differential sampling can separate loosely, moderately and tightly held particles.

On the contact surfaces of footwear loosely, moderately and strongly held particle fractions were separated and analyzed in an effort to detect different particle signals.Three environmental exposure sites were chosen to have different, characteristic particle types (soil minerals). Shoes of two types (work boots and tennis shoes) were tested, accumulating particles by walking 250 m in each environment. Some shoes were exposed to only one environment; others were exposed to all three, in one of six different sequences.Sampling methods were developed to separate particles from the contact surface of the shoe based on how tightly they were held to the sole. Loosely held particles were removed by walking on paper, moderately held particles were removed by electrostatic lifting, and the most tightly held particles were removed by moist swabbing.The resulting numbers and types of particles were determined using forensic microscopy. Particle profiles from the different fractions were compared to test the ability to objectively distinguish the order of exposure to the three environments.Without exception, the samples resulting from differential sampling are dominated by the third site in the sequential footwear exposures. No noticeable differences are seen among the differential samplings of the loosely, moderately and strongly held particles: the same overwhelming presence of the third site is seen. It is clear from these results (1) that the third (final) exposure results in the nearly complete removal of any particles from prior exposures, and (2) that under the experimental conditions loosely, moderately and strongly held particles are affected similarly, without any detectable enrichment of the earlier exposures among the more tightly held particles.These findings have significant implications for casework, demonstrating that particles on the contact surfaces of footwear are rapidly lost and replaced.