Hair & Fiber Evidence
Although it is often depicted on TV shows, this is not as common a piece of evidence as you may think. Depending on the context of the case, this type of evidence may have a limited evidential value. However, in some cases this can be a very key piece of evidence. As such, it should never overlooked at the beginning of an investigation. Even though this type of evidence will often be more time consuming and expensive to analyze than other evidence, it still may have significant value!
Where to Begin?
To the unaided eye, hairs and fibres may look virtually indistinguishable. However, when examined closely -- under magnification, using reflected and transmitted light -- it can be very easy to distinguish a hair from a fiber. And, if it is a hair, it may be easy to determine the type of animal it came from.
The shaft of the hair will have three basic components: Cuticle, Cortex, and Medulla. From the Cuticle and Medulla, it may be easy to determine what species the hair originated from.
Cuticle & Medulla Patterns
Both have discernible patterns which can help distinguish between differing species
3 basic types:
4 basic types:
Note: Both Cuticle & Medulla structure can vary in the types of hairs an animal may have. Animals generally have more than one type of hair.
What About Fibers?
Like Hair samples, fibers may be distinguished by their physical characteristics as well, when examined under a microscope using reflected light.
Three most common types:
Cotton - has a flat ribbon-like structure with convolutions. It is thin, and close to transparent, and often with a high sheen. Cross-section is kidney shaped.
Polyester - man-made fiber, with an unnaturally smooth and uniform in diameter, in a rod-like appearance. Cross-section is circular.
Wool - real wool is derived from animal hairs. As such, you will see the cuticle and medulla patterns depending on the animal hair used.
Can Hairs or Fibers be used for Identification?
Generally speaking, not usually. It is uncommon for hair or fiber evidence to provide a unique identification. Quite often they only provide evidence with class characteristics, which can narrow down to a group.
Hair may offer more in regard to identification, for example: the hair shaft will have mitochondrial DNA, which can narrow to a maternal line. The only way hair can be used to uniquely identify is if nuclear DNA is attached (root bulb). Hair goes through three growth stages:
Anagen - initial growth phase,
Catagen - transition phase,
Telogen - hair naturally shed.
Nuclear DNA is only likely attached to hair which was forcefully removed during the Anagen growth phase. The root shrinks and is lost during the Catagen and Telogen phases.
Fibers will often only ever offer class characteristic type evidence. As fibers often come from items which are mass produced, they do not often provide as much as a hair might.
However, the evidence provided by hairs or fibers may be a circumstantial link that has a great deal of evidential value. There may be aspects of the hair or fiber which provides more specific class characteristic type evidence. And, circumstantial links will likely have more value, as the quantity of existing links are increased.