This article incorporates, in modified form, material from the not-yet-published Illustrated Guide to Forensics Investigations: Uncover Evidence in Your Home, Lab, or Basement.
Along with soil, hairs and fibers are the most common forms of trace evidence processed by forensics labs. In nearly all violent crimes, hairs and fibers are transferred from the criminal to the victim or the crime scene, and vice versa.
Hairs and fibers are easily transferred from people to people, people to objects, and objects to people, and they often cling tenaciously to the new environment. Hairs and fibers do not degrade quickly, which means that known specimens collected long after the fact can be compared successfully against questioned specimens collected weeks, months, or even years before at a crime scene. Hairs and fibers are both ubiquitous and easily overlooked, so it’s nearly impossible for a criminal to remove all of them from the scene, even if he is sufficiently well-informed to recognize their potential evidentiary value.
Because of their ubiquity and the diversity of their individual characteristics, hairs and fibers are an excellent form of class evidence. Hair may sometimes be individualized by DNA analysis, but even if a specimen cannot be individualized it may be possible to categorize it quite specifically on the basis of its many class characteristics. Such evidence, although not conclusive, may be quite useful to establish guilt or innocence.
Although professional forensics labs often use instrumental analysis techniques to establish matches conclusively, wet chemistry tests and microscopy remain important techniques for hair and fiber analysis, both for initial screening and for establishing matches that will be used in court testimony. In this chapter, we’ll perform many of the wet-chemistry and microscopic tests that real forensics labs do every day.
A Day in the Life of a T-shirt
If you want to do a real-world forensic hair and fiber analysis, begin with a new t-shirt, still in the plastic wrap. One day, unwrap the t-shirt and wear it all day long. Take notes throughout the day of the places you visit, the people and animals you encounter, where you sit, and so on.
At the end of the day, remove the t-shirt and place it on a clean surface. (A trash bag fresh from the box provides an uncontaminated work surface.) Under a strong light, use your magnifier or loupe to examine the entire surface of the t-shirt, front and back. Use forceps or tweezers to remove any hairs or fibers you find, and transfer them to a collection envelope.
Examine all of the questioned hair and fiber specimens using the procedures described in this section, and try to determine where they originated. When we tried this experiment, we found numerous hairs from ourselves and our dogs, fibers from our sofa, car seats, and office chairs, and many fibers for which we couldn’t establish an origin, including several human and animal hairs and various fabric fibers.