Weathering the storm: how weather conditions impact the persistence of fiber evidence

Photo Credit: Kristyn Ramsey

European forensic scientists investigate how outdoor weather conditions impact fiber loss on static textiles, giving more quantifiable information for investigators to direct their forensic resources.

Fiber evidence can establish strong links between suspects, victims and crime scenes by the transfer of material from one to another. This kind of evidence was crucial to the capture and conviction of Colorado’s Corridor Killer.  However, fibers are only valuable if they stay after the initial transfer, quantified as persistence. Stormy weather destroys various types of evidence including DNA, but would these conditions have the ability to remove important fiber evidence on clothing after close contact transfer? To answer this question, researchers at the University of Lausanne and the National Institute of Criminalistics and Criminology analyzed fiber persistence after leaving t-shirts in various mountainous, rainy and windy conditions in Switzerland.

Textile fibers often play a key role in forensic investigations as they connect suspects, victims, and crime scenes days or even years after their occurrence (Fig. 1). The amount of fibers exchanged between these players depends on factors during the crime, such as clothing material (e.g. cotton fibers onto polyester jacket) and the contact between them (e.g. long or short, tight or loose), and after the crime, such as the time to collect the evidence and the environment between occurrence and recovery. Fortunately, research and investigator experience has shown there is little loss of transferred fibers when collected from static material, such as fibers on a shirt worn by a dead body. However, exposure to the outside and stormy weather may significantly hasten this fiber loss. This motivated the authors of this study to quantitatively evaluate the effects of several weather conditions on fiber transfer to a clothed, deceased body.

Figure 1: Displays the process of fiber transfer during contact events. Photo Credit (edited): Pinterest

The scientists rubbed together a polyester pink hoodie and a black cotton t-shirt to transfer the pink fibers (Fig. 2) and then left the t-shirt outside, somewhere on the forested terrain of Zurich, Switzerland. They also put a pink-fiber-covered t-shirt indoors to see if they could replicate their outside studies using a “weather machine” with electric tower fans, vertical sprinklers and tilted floors (to simulate mountain inclines). Comparing the controlled laboratory setting to the true outdoors would not only reveal quantifiable information on fiber persistence, but also determine if other scientists could simulate weather impacts to forensic evidence indoors.

Figure 2: Photographic examples of pink donor fibers on the black recipient tees. Photo Credit: Prod’Homa et al, 2021

Their “true” outdoor experiment that wind had the most significant impact on persistence, with high wind (over 9 m/s) removing the pink fibers exponentially while medium wind conditions (between 3 and 5 m/s) led to minimal fiber loss. This foundational knowledge helps forensic investigators better direct their efforts and not waste precious time searching for fibers when high winds were reported. While wind and rain together produced much less loss than wind alone – an interesting and positive find – their results varied unexpectedly with no clear trend. Increased ground inclination was correlated to greater fiber loss in the outdoor setting and time was found to be important in that it allowed for more activity, whether weather or human/animal influence to occur.

Figure 3: The experimental design of the weather machine and settings used for each test. Photo Credit: Prod’Homa et al, 2021

In comparison to the true outdoors, the researchers found that for their weather machine, time itself was a nonfactor. Much like when leaving a shirt in a dresser drawer, exposing the fibers to nothing over time will produce nothing, which strongly demonstrated how certain aspects of the weather machine could not capture what occurred outside. This was further seen with their fan-produced wind, which minimally impacted the pink fiber’s persistence, while their sprinkler-produced rain had the greatest effect. Interestingly, rain and wind together, like in the true outdoors, only marginally affected fiber persistence and in some cases caused a decrease in fiber loss. Researchers theorized that this might be due to the wind drying some of the precipitation and reducing its overall force of impact. The indoor simulation resulted in a pink fiber loss of no more than 10% for the wind, rain or time factors. The inability to control for unidentified external factors in the outdoor setting could have caused more fibers to be lost, such as animal and human manipulation.

This study shows how wind can dramatically impact evidence degradation and persistence of transferred fibers, highlighting the importance of recovering evidence as quickly as possible.  Rain proved to have the largest impact in the simulation, but very minimal in true outdoor conditions.  Surprisingly, rain and wind in combination had only a slight impact on fiber loss for both experiments.  Researchers noted that time was important as it increased the exposure of the textile to the elements and potential animal or human influence. While the outcomes of this study may appear straight-forward in some aspects and surprising in others, quantifying our “common sense” strengthens the reliability of forensic evidence presented in court, instead of shoddy convictions based on false practices. Fiber evidence remains an important piece of information that can help investigators link crimes to suspects and suspects to victims, but must be analyzed with scientifically verified methods and framed within accurate statistical contexts.

Title: Fibre persistence on static textiles under outdoor conditions
Authors: Alizée Prod’Homa, Denis Wernera, Laurent Lepotb, Geneviève Massonnet
Journal: Forensic Science International
Year: 2021

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